Special Issue Article

Advances in Mechanical Engineering2016, Vol. 8(6) 1–9� The Author(s) 2016DOI: 10.1177/1687814016654045aime.sagepub.com

Effects of cavitation on performance ofautomotive torque converter

Jaewon Ju1, Jaedeuk Jang2, Minsuk Choi2 and Je Hyun Baek1

AbstractCavitation is a phenomenon whereby vapor bubbles of a flowing liquid are formed in a local region where the pressureof the liquid is below its vapor pressure. It is well known that cavitation in torque converters occurs frequently when acar with an automatic transmission makes an abrupt start. Cavitation is closely related to a performance drop and noisegeneration at a specific operating condition in a car and a torque converter itself. This study addressed the relationbetween cavitation and performance in an automotive torque converter in a quantitative and qualitative manner usingnumerical simulations. The cavitation was calculated at various operating conditions using a commercial flow solver withthe homogeneous cavitation model, and the torque converter performance was compared with the experimental data.Numerical results well match to the data and indicate that the cavitation causes significant performance drop, as thepump speed increases or both speed ratio and reference pressure decrease.

KeywordsTorque converter, cavitation, C-factor, torque ratio, separation

Date received: 26 October 2015; accepted: 19 May 2016

Academic Editor: Yuning Zhang

Introduction

A torque converter is a fluid coupling device used fortransferring rotating power from an engine to an auto-matic transmission. A torque converter is usually com-posed of three parts: pump driven by the engine,turbine driving the transmission, and stator mountedon an overrunning clutch positioned between the pumpand turbine. An internal fluid circulating through thethree components can vaporize locally to form bubblesat a specific operating condition where the static pressuregoes below its vapor pressure. Although the operatingcondition stays the same for a while, cavitation bubblesextend to a considerable part of a flow passage, causingsignificant performance reduction, severe noise, andvibration in a car and a torque converter itself. Becausethese results from the cavitation have a negative effect onfuel efficiency and driving comfort, many researchers areinvestigating the cavitation in a torque converter and try-ing to find an effective method for suppressing it.

Most of the studies have been focused on predictingthe occurrence of cavitation at stall by experimentalmethods. Anderson et al.1 measured the static pressureon the pump blade to predict the initiation of cavita-tion. Mekkes et al.2 defined a fluctuating cavitationnumber based on the static pressure on the suction side

1Department of Mechanical Engineering, Pohang University of Science

and Technology (POSTECH), Pohang, Republic of Korea2Department of Mechanical Engineering, Myongji University, Yongin,

Republic of Korea

Corresponding authors:

Minsuk Choi, Department of Mechanical Engineering, Myongji University,

Yongin 17058, Republic of Korea.

Email: mchoi@mju.ac.kr

Je Hyun Baek, Department of Mechanical Engineering, Pohang University

of Science and Technology (POSTECH), Pohang 37673, Republic of

Korea.

Email: jhbaek@postech.ac.kr

Creative Commons CC-BY: This article is distributed under the terms of the Creative Commons Attribution 3.0 License

(http://www.creativecommons.org/licenses/by/3.0/) which permits any use, reproduction and distribution of the work without

further permission provided the original work is attributed as specified on the SAGE and Open Access pages (https://us.sagepub.com/en-us/nam/

open-access-at-sage).

of the stator blades to predict the onset of cavitation.Kowalski et al.3,4 used nearfield acoustical measure-ments with a high pass filter for detecting the cavitationand suggested a correlation based on their experimentalresults to predict the cavitation. Robinette et al.5–7 per-formed a series of experiments for several torque con-verters with different sizes and different shapes andthen suggested a cavitation prediction model applicableto every torque converter. Based on the experimentaldata of various geometries of torque converters, in par-ticular, Walber et al.8 developed a method to determinethe speed ratio (SR) of cavitation desinence. As statedabove, the experiments for the cavitation in torque con-verters can provide useful information about the exis-tence of cavitation bubbles based on the measurednoise levels, vibrations, and torques of each compo-nent. However, it is difficult to construct the shape andsize of the cavitation bubbles and to point out theirexact locations.

In computational researches, Dong et al.9 foundfrom their numerical results that the cavitation on theleading edge of the stator grows in size with theincreasing pump speed, resulting in large performancedrop in a torque converter. Numerical methods havealso been used for researchers to advance their under-standing of complex three-dimensional (3D) flow fea-tures such as secondary flow10 and for designers toanalyze the performance of a torque converter and todesign its efficient components.11 Watanabe et al.12

predicted cavitation behaviors in an automotive torqueconverter at the stall condition with numerical simula-tions and analyzed the relationship between cavitationand vibration. They found that the mechanical vibrationsincrease with the development of cavitation but decreasewith excessive development of cavitation thereafter.

Numerical simulation for cavitation in torque con-verters is still a developing field, although flow simula-tion has been routinely performed to understand acomplex flow in a torque converter. Moreover, most of

the numerical studies have been focusing on the predic-tion of cavitation at the stall condition only. It is neces-sary to test numerical methods including cavitationmodels in various operating conditions of torque con-verters for their validation. This article covers extensivenumerical simulations for various operating conditionswith different pump speeds and SRs between the pumpand the turbine, comparisons of numerical results withexperimental data, and analysis about the effects ofcavitation on the performance in an automotive torqueconverter.

Numerical model

Computational domain and mesh

The numerical results in this work were for geometry ofan automotive torque converter, which is mainlymounted on current sedans with an automatic transmis-sion. All geometrical factors are typical of a modernautomotive torque converter. The computationaldomain is assumed to be a closed system consisting of apump, stator, and turbine, which is slightly differentfrom the experimental test facility. In the test facility,there exists a leakage of automatic transmission fluid(ATF) between the three components enclosed in thefront cover. Computational meshes for the three com-ponents were generated by a commercial mesh genera-tor, ANSYS ICEM-CFD, as shown in Figure 1.Hexahedral structured mesh is used for the pump andturbine, and hexahedral unstructured mesh is adoptedfor the stator due to its complex shape. A single passageof each component has about 200,000 nodes, and thewhole computational domain has about 600,000 nodes.

Computational method

A commercial flow solver, ANSYS CFX 14.5, has beenused for the numerical simulations of the internal flowin a torque converter. This flow solver has already been

Figure 1. Computational mesh for each component: (a) pump, (b) turbine, and (c) stator.

2 Advances in Mechanical Engineering

validated with different torque converters bySchweitzer and Gandham11 and Lee et al.13 The con-vection terms in the 3D compressible Navier–Stokesequations were discretized by a high-resolution scheme,which is a blend of a first-order upwind differencescheme and a second-order central difference scheme.The spatial derivatives of the diffusion terms were cal-culated by shape functions formulated by the finite ele-ment method. A first-order backward Euler timestepping was applied for obtaining steady solutions.Each numerical solution was considered to be con-verged to a steady solution when the residuals in mass,momentum, and turbulence variables were \1024. Thek–v shear stress transport (SST) model has been usedto obtain turbulent viscosity because the internal flowhas complex 3D characteristics. For implementing cavi-tation numerically, the homogeneous multiphase modelwithout any free surface model is applied to this studybecause the vapor velocity is assumed to be the same asthe liquid velocity. The mass transfer between thevapor and the liquid is calculated based on theRayleigh–Plesset cavitation model. The saturation pres-sure, provided by the ATF maker, is set to be0.035 bar, while all the other empirical parameters ofthe cavitation model are the default values of ANSYSCFX 14.5 as listed in Table 1. The advantage of thismodel is its ability to simulate cavitation andphase change correctly. The detailed description for the

numerical methods and the cavitation model can befound in ANSYS CFX 14.5 manuals.14

In the simulation of a torque converter, there are noinlet and outlet conditions because it is assumed to be aclosed system. The no-slip condition has been used onthe wall surfaces, and the periodic condition has beenemployed because only one flow passage is used in eachcomponent. On the interface between moving and sta-tionary parts, the mixing plane was employed for steadysimulations with and without the cavitation model.

Operating conditions

In the simulations for cavitation, it is important to seta proper reference pressure because the evaporation ofa working liquid is inversely proportional to the refer-ence pressure. In the experimental facility, it is very dif-ficult to measure the static pressure inside the pump,stator, and turbine due to their rotation. The torqueconverter was pressurized by a transmission pump, andthe charge pressure was measured at the inlet, whichwas applied as the reference pressure in the simulations.Only torque of the pump and turbine was measuredfrom the test facility, as it was difficult to determinewhether the cavitation occurred inside the torque con-verter or not.

In this study, a total of 24 operating conditions weresimulated for capturing cavitation in the torque conver-ter with different reference pressures, pump speeds,and SRs between the pump and the turbine. Thedetailed conditions for the simulations are summarizedin Table 2.

Computational results

Grid independence test and validation

The performance of a torque converter is defined bythe rotational speeds and torques of the pump and theturbine. SR and torque ratio (TR) are the ratio of theturbine relative to the pump in speed and torque asdefined in equations (1) and (2). Especially, in case ofSR=0, the operating condition is called ‘‘stall.’’ In anautomobile, this condition occurs when the transmis-sion is in gear, but the vehicle cannot move due to thebrake. C-factor (CF; torque capacity) is directly pro-portional to the pump torque but inversely propor-tional to the square of the pump rotational speed asdefined in equation (3)

SR=NT

NP

ð1Þ

TR=TT

TP

ð2Þ

Table 1. Parameters of Rayleigh–Plesset cavitation model.

Parameter Value

Saturation pressure 0.035 barMean nucleation site diameter 2 3 1026 mCavitation condensation coefficient 0.01Cavitation vaporization coefficient 50Maximum density ratio 1000Nuclei volume fraction 5 3 1024

Cavitation rate under-relaxation factor 0.25

Table 2. Operating conditions for cavitation simulations.

Referencepressure (bar)

Pumpspeed (r/min)

Speed ratio

4 1930 0 0.3 – –2300 0 0.3 – –2800 0 0.3 – –

5 1600 0 0.3 0.5 0.71930 0 0.3 0.5 0.72300 0 0.3 0.5 0.7

6 1700 0 0.3 – –2300 0 0.3 – –2800 0 0.3 – –

Ju et al. 3

CF=TP kgfmð Þ

N 2P r=min2ð Þ 3 106 ð3Þ

Prior to validation of numerical results, a grid inde-pendence test has been performed for three grids withdifferent resolutions as listed in Table 3. The referencepressure, pump rotational speed, and SR are 5 bar,1930 r/min, and 0.3, respectively. TR and CF of eachgrid have been compared with one another, and thecomparison result is shown in Figure 2. The differencesin TR and CF between grids 2 and 3 are \0.34% and0.43%, respectively; hence, grid 3 was selected for fur-ther analysis.

The computational results with and without thecavitation model were compared with the experimentalmeasurements for validation. For a fixed reference sta-tic pressure of 5 bar, variations of TR and CF alongthe SR were calculated in three different pump speeds,and the results are shown in Figure 3. In the experi-mental data, CF increases steadily, makes its maximumat SR=0.6, and decreases rapidly thereafter.Theoretically, the torque converter can produce itsmaximum torque at stall if the input power is suffi-ciently applied. In all the three cases, however, the max-imum torque was obtained around SR=0.6, implyingthat there is a large loss in transferring power at stall.By the way, TR approaches to 1 as SR increases. In allthe computational cases, TR is well matched to theexperimental values regardless of the existence of thecavitation model. However, CF is significantly affectedby the existence of the cavitation model. Without the

cavitation model, the computations considerably over-predict CF in comparison with the experiments whenSR is \0.5. Above 0.5 of SR, the computations predictCF correctly because cavitation disappears in the flowfield for higher SR. With the cavitation model, CFsagree well with the experimental measurements over allpump speeds in comparison with the CFs without thecavitation model. There are two important things topoint out. First, a cavitation model is essential in theflow simulation in order to predict CF correctlybecause cavitation causes a large loss in power transferfrom the pump to the turbine when SR has low values.Second, it is worth noting that the efficiency of powertransfer could be greatly increased if the cavitation iscontrolled and suppressed with a proper operating con-dition or a good blade shape.

As shown in Figure 3(a) and (b), however, there arestill some differences even if the cavitation model isemployed. This difference can be caused by three fac-tors. First, the computational domain has beenassumed to be a closed system so that the simulationscannot consider the leakage between the three compo-nents. The amount of the leakage might be changeddepending on the speeds of the pump and turbine.Second, the reference pressure measured indirectly atthe inlet in the experiment might not be the same as thestatic pressure inside the torus of the torque converterdue to the pressure loss. Third, default values for theparameters of the cavitation model might not be theoptimal values for ATF because the parameters havegenerally been determined based on water experiments.

Flow analysis

Figure 4 shows the extent of cavitation with differentSRs at the same reference pressure and pump speed. Atpref=5bar and NP=1930 r/min, the cavitation occursonly on the stator surface and not on the pump and theturbine surfaces. At stall, the cavitation appears fromthe mid-span to the shell at the leading edge of the sta-tor. As SR increases, the cavitation shrinks in the span-wise direction and finally disappears on the statorsurface when SR is .0.5, as shown in Figure 4(b) and(c). In consideration with Figure 3(b), CF increasesfrom stall to SR=0.5 because the size of the cavitationbubbles decreases.

Figure 5 shows velocity fields at the mid-span of thestator with different SRs. The computational conditionis the same as the case in Figure 4. The incidence anglehas its maximum value at stall due to the motionlessturbine, causing a large separation on the suction sur-face from the leading to the trailing edges. As SRincreases, the incidence angle decreases, and the smallincidence delays the onset of the separation to a furtherdownstream position. In particular, at SR=0.3, theflow is separating behind the leading edge. At

Table 3. Three different grids for grid independence test.

Grid no. Pump Turbine Stator

1 56,896 56,884 73,9662 109,080 123,405 129,3763 216,720 204,129 264,019

Figure 2. Grid independence test at pref = 5 bar, NP = 1930 r/min, and SR = 0.3.

4 Advances in Mechanical Engineering

Figure 3. Torque ratio and C-factor for different pump speeds: (a) pref = 5 bar, NP = 1600 r/min; (b) pref = 5 bar, NP = 1930 r/min; and(c) pref = 5 bar, NP = 2300 r/min.

Figure 4. Cavitation on stator surface with different speed ratios (pref = 5 bar and NP = 1930 r/min): (a) stall, (b) SR = 0.3, and (c)SR = 0.5.

Ju et al. 5

SR=0.5, the flow is almost attached on the suctionsurface, but a small separation region still exists nearthe trailing edge. When SR is .0.5, no separation existson the stator surface. In comparison with Figure 4, theseparation on the leading edge of the stator is closelyrelated to the onset of cavitation. The flow acceleratesaround the leading edge, and the local static pressuredrops significantly, while the static pressure recoversnear the trailing edge on the suction surface. Therefore,the static pressure near the leading edge has its mini-mum value in the torque converter. When the flow isseparating due to a large incidence angle, the local pres-sure decreases further, and the possibility of cavitationincreases significantly. When the local static pressure isbelow the vapor pressure of ATF, cavitation initiates.This is why the cavitation occurs easily on the leadingedge rather than on the trailing edge of the stator.

In order to analyze a relationship between cavitationand separation on the leading edge, velocity fields alongthe span are shown in Figure 6. At the same operatingcondition as in Figure 4(b), a large separation occurson the suction surface of the stator near the shell(span=0.9) due to the large incidence angle, which iscaused by the slow turbine motion at SR=0.3. At themid-span (span=0.5), the incidence angle becomessmaller, and the separation also decreases in size. In

particular, the flow around the leading edge of the sta-tor is attached on the suction surface due to thedecreased incidence, and the separation starts behindthe leading edge. Near the core (span=0.1), the flow iscompletely attached on the stator surface, and there isno separation. Based on this result, it is evident that theextent of cavitation is closely related to the separationon the leading edge of the stator. The possibility ofcavitation increases significantly whenever the separa-tion occurs and causes low-pressure region around theleading edge.

Figure 7 shows the extent of cavitation at stall con-dition with different pump speeds. At NP=1600 r/min,thin cavitation covers the stator surface from the mid-span to the shell, blocking the internal flow passageand dropping CF. When NP reaches 1930 r/min, thecavitation expands forward the core, and its thicknessbecomes larger, making the blockage increase further.At 2300 r/min, the whole leading edge of the stator iscovered with the thicker cavitation. In addition,another cavitation is observed at the pump inlet andthe turbine inlet, even though its size is very small. As aresult, CF decreases as the pump speed increases, asshown in Figure 8 except for point A. The CF variationalong the pump speed at stall is well matched to theresult of Dong et al.9 These results show that theabrupt start of a car with a high engine speed can cause

Figure 5. Velocity vector at stator mid-span (pref = 5 bar andNP = 1930 r/min): (a) stall, (b) SR = 0.3, (c) SR = 0.5, and (d)SR = 0.7.

Figure 6. Velocity fields at different spans of the stator(pref = 5 bar, NP = 1930 r/min, and SR = 0.3): (a) span = 0.9, (b)span = 0.5, (c) span = 0.1, and (d) three planes.

6 Advances in Mechanical Engineering

large cavitation in the torque converter and also canmake its fuel efficiency worse.

A torque converter is generally sealed up with a highreference pressure to prevent the cavitation. To analyzethe effect of the reference pressure on cavitation, thecomputation was conducted with different pressures asshown in Figure 9. At pref=4bar, cavitation covers allspan at the leading edge of the stator, and it alsoappears near the shell and core at the trailing edge. Asthe reference pressure increases to 5 bar, the cavitationat the leading edge shrinks a little in the spanwise direc-tion, and the cavitation at the trailing edge disappears.When the reference pressure increases further to 6 bar,

the cavitation still exists on the stator surface, althoughits size becomes smaller. As a result, the CF increaseswith the increasing reference pressure as shown inFigure 10. However, this result shows that the high ref-erence pressure is not a perfect method to suppress thecavitation, even though it relieves cavitation phenom-ena in the torque converter and reduces their negativeeffects on the performance.

Conclusion

Using an advanced numerical model, cavitation in atorque converter was investigated in detail for differentoperating conditions. The following conclusions can bemade from an inspection of the numerical results.

1. The computed results with a cavitation modelagreed well with the experimental data in the CFand the TR between the pump and the turbine.However, the computation without the cavitationmodel overpredicted CF significantly in case ofSR\ 0.5. Therefore, the cavitation model isessential in the flow simulation of a torque con-verter to predict its performance accurately.

2. In all pump speeds, the cavitation occurs at therange of 0–0.5 of SR. Due to the slow turbine, theincidence angle at the leading edge of the statorincreases significantly, inducing a large separation

Figure 7. Distribution of cavitation at different pump speeds (pref = 5 bar and stall): (a) NP = 1600 r/min, (b) NP = 1930 r/min, and (c)NP = 2300 r/min.

Figure 8. C-factor distribution along the pump speeds at stall.

Ju et al. 7

around the leading edge and finally causing thecavitation. It should be noted that the leadingedge of the stator near the shell is the most sus-ceptible part to cavitation in a torque converter.

3. At stall condition, the cavitation region expandsat the leading edge of the stator in the spanwisedirection as the pump speed increases, causingCF to drop significantly. This implies that theabrupt start of a car could reduce fuel efficiencyconsiderably.

4. To prevent the onset of cavitation, a torque con-verter is sealed up with a high reference pressure.The high reference static pressure consideredhere is helpful to decrease the cavitation in sizeand to increase CF at stall, but it is insufficientto remove cavitation completely.

5. This result suggests that some different tech-niques are required to suppress the cavitation in atorque converter. Based on this work, a study isbeing carried out to find the optimized shape ofthe stator with high resistance against cavitation.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest withrespect to the research, authorship, and/or publication of thisarticle.

Funding

The author(s) received no financial support for the research,authorship, and/or publication of this article.

References

1. Anderson C, Zeng L, Sweger P, et al. Experimental

investigation of cavitation signatures in an automotive

torque converter using a microwave telemetry technique.

Int J Rot Mach 2003; 9: 403–410.2. Mekkes J, Anderson C and Narain A. Static pressure

measurement and cavitation signatures on the nose of a

torque converter’s stator blades. In: International sympo-

sium on transport phenomena and dynamics of rotating

machinery (ISROMAC 10), Honolulu, HI, 7–11 March

2004, paper no. 10-2004-035.3. Kowalski D, Anderson C and Blough J. Cavitation detec-

tion in automotive torque converters using nearfield

acoustical measurements. SAE paper 2005-01-2516, 2005.

Figure 9. Distribution of cavitation at different reference pressures (NP = 2300 r/min and stall): (a) pref = 4 bar, (b) pref = 5 bar, and (c)pref = 6 bar.

Figure 10. C-factor distribution along the reference pressuresat stall and NP = 2300 r/min.

8 Advances in Mechanical Engineering

4. Kowalski D, Anderson C and Blough J. Cavitation pre-

diction in automotive torque converters. SAE paper

2005-01-2557, 2005.5. Robinette D, Anderson C, Blough J, et al. Characterizing

the effect of automotive torque converter design para-

meters on the onset of cavitation at stall. SAE paper

2007-01-2231, 2007.6. Robinette D, Schwitzer J, Maddock D, et al. Predicting

the onset of cavitation in automotive torque converters:

part 1—designs with geometric similitude. Int J Rot

Mach 2008; 2008: Article ID 803940 (8 pp.).7. Robinette D, Schwitzer J, Maddock D, et al. Predicting

the onset of cavitation in automotive torque converters:

part 2—a generalized model. Int J Rot Mach 2008; 2008:

Article ID 312753 (8 pp.).8. Walber C, Blough J, Anderson C, et al. Predicting cavita-

tion desinence in automotive torque converters. In: 26th

international conference on noise and vibration engineering

(ISMA 2014), Leuven, 15–17 September 2014, pp.4079–

4094.9. Dong Y, Korivi V, Attibele P, et al. Torque converter

CFD engineering part 2: performance improvement

through core leakage flow and cavitation control. SAE

paper 2002-01-0884, 2002.10. Abe K, Kondoh T, Fukumura K, et al. Three-dimen-

sional simulation of the flow in a torque converter. SAE

paper 910800, 1991.

11. Schweitzer J and Gandham J. Computational fluid

dynamics in torque converters: validation and applica-

tion. Int J Rot Mach 2003; 9: 411–418.12. Watanabe S, Otani R, Kunimoto S, et al. Vibration char-

acteristics due to cavitation in stator element of automo-

tive torque converter at stall condition. In: ASME 2012

fluids engineering division summer meeting (FEDSM

2012), Rio Grande, Puerto Rico, 8–12 July 2012, paper

no. FEDSM2012-72418. New York: ASME.13. Lee C, Jang W, Jang J, et al. Three dimensional flow field

simulation to estimate performance of a torque converter.

SAE paper 2000-01-1146, 2000.14. Ansys Inc. Ansys CFX-solver theory guide and Ansys CFX

modeling guide (Ansys documentation for release 14.5).

Canonsburg, PA: Ansys, Inc., 2012.

Appendix 1

Notation

NP pump rotational speedNT turbine rotational speedpref reference pressureTP pump torqueTT turbine torque

Ju et al. 9