An Analysis of Drag Force Components and Structures in the...

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17 th International Symposium on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 07-10 July, 2014 - 1 - An Analysis of Drag Force Components and Structures in the Wake of a Detailed Road Vehicle Model by Means of SPIV Robin Placzek 1,* , Peter Scholz 1 , Carsten Othmer 2 1 Istitute f Fuid echaics Techische Uiversität Brauschweig Geray 2 V!swage AG Grup Research Vehice Techgy Wfsburg Geray ’ crrespdet authr r.pac)e!@tu+bs.de Abstract The three+diesia fw fied f a 14 scae rad vehice de has bee ivestigated. Due t the de geetry icudig a detaied uderbdy whees.wheehuses ad irrrs the fw fied is affected by severa iterferig effects. Typica fr a buff bdy i grud effect fw separati at the rear resuts i the frati f a characteristic recircuati area behid the de. This wa!e has a a/r ipact the uter fw as we as the tta aerdyaic perfrace f the vehice. Aay)ig the diat fw ad vrtex structures f the wa!e ad specifyig their particuar ipact drag was the ai b/ective f this study. The fw fied was ivestigated usig Sterescpic 3C2D+DPIV frce ad pressure easureets as we as i visuai)ati. The experieta data prvides a iprtat isight it the iteracti f pressure iduced separati ad gitudia vrtices f this reaistic rad vehice square bac! geetry. As secdary effects such as vrtices appearig i the wa!e ca be traced bac! t their geetric rigi. A evauati f the idividua drag cpets is carried ut. The experieta resuts are the cpared t a usteady uerica siuati usig 6peF6A ® . Sice a crrect uerica predicti f the itegra vaues ad the des fw fied strgy depeds the accurate predicti f the fw separati ad the three diesia iteracti betwee wa!e ad uter fw the SPIV data ctais vauabe data t vaidate the siuati. The agai the uerica data f the three diesia siuati ca be used t idetify ad trace bac! fw structures that appear withi the tw diesia PIV paes. 1 Itrducti Due t icreasig iprtace f fue efficiecy ad further reducti f C62 eissis the aerdyaic perfrace f prducti cars ctiues t be a !ey aspect i research ad devepet. After prgress has aready bee ade especiay i the fied f ightweight structures the aerdyaic ptetia fr decreasig fue csupti reais. Fr iprvig the aerdyaic perfrace a better uderstadig f a rea fw fied is adatry. This fied arud a car beig a buff bdy i grud effect ca be characteri)ed by three+diesia fw separati r reattachet ad vrtex fw. T a high degree the drag f a buff bdy depeds the wa!e structure 7Huch]. Depedig the rear geetry the wa!e itsef is a superpsiti f ift iduced gitudia vrtices f variabe itesity ad a arge separated recircuati fied the s caed dead water. It is a highy turbuet dissipative fied icudig usteady vrtex fw ad brea!dw 7Eber)]. The uderbdy whees ad wheehuses ca ctribute as uch as 50% t the tta drag sice they have a strg ipact the ai fw features as we. Thus they cat easiy be egected. Geeric cfiguratis ca ead t a very differet fw fied which a!es it difficut t trasfer resuts i!e depedecies ad paraeters t re cpex cfiguratis such as prducti cars. The appearig structures withi this cpex fw arud the detaied de which is preseted ca y be resved with a high+quaity vecity fied easureet techique such as Sterescpic Partice Iage Vecietry (SPIV). This study reates the resved wa!e structures t their geetric rigi ad t their ctributi t vera drag. The data is used t vaidate the

Transcript of An Analysis of Drag Force Components and Structures in the...

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17th International Symposium on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 07-10 July, 2014

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An Analysis of Drag Force Components and Structures in the Wake of a

Detailed Road Vehicle Model by Means of SPIV

Robin Placzek1,*, Peter Scholz1, Carsten Othmer2

1� I�stitute �f F�uid �echa�ics� Tech�ische U�iversität Brau�schweig� Ger�a�y

2� V��!swage� AG� Gr�up Research� Vehic�e Tech����gy� W��fsburg� Ger�a�y

' c�rresp��de�t auth�r� r.p�ac)e!@tu+bs.de

Abstract The three+di�e�si��a� f��w fie�d �f a 1�4 sca�e r�ad vehic�e ��de� has bee� i�vestigated. Due t� the ��de� ge��etry i�c�udi�g a detai�ed u�derb�dy� whee�s.whee�h�uses a�d �irr�rs� the f��w fie�d is affected by severa� i�terferi�g effects. Typica� f�r a b�uff b�dy i� gr�u�d effect f��w separati�� at the rear resu�ts i� the f�r�ati�� �f a characteristic recircu�ati�� area behi�d the ��de�. This wa!e has a �a/�r i�pact �� the �uter f��w as we�� as �� the t�ta� aer�dy�a�ic perf�r�a�ce �f the vehic�e. A�a�y)i�g the d��i�a�t f��w a�d v�rtex structures �f the wa!e a�d specifyi�g their particu�ar i�pact �� drag was the �ai� �b/ective �f this study. The f��w fie�d was i�vestigated usi�g Stere�sc�pic 3C2D+DPIV� f�rce a�d pressure �easure�e�ts� as we�� as �i� visua�i)ati��. The experi�e�ta� data pr�vides a� i�p�rta�t i�sight i�t� the i�teracti�� �f pressure i�duced separati�� a�d ���gitudi�a� v�rtices �f this rea�istic r�ad vehic�e square bac! ge��etry. A�s� sec��dary effects such as v�rtices appeari�g i� the wa!e ca� be traced bac! t� their ge��etric �rigi�. A� eva�uati�� �f the i�dividua� drag c��p��e�ts is carried �ut. The experi�e�ta� resu�ts are the� c��pared t� a� u�steady �u�erica� si�u�ati�� usi�g 6pe�F6A�®. Si�ce a c�rrect �u�erica� predicti�� �f the i�tegra� va�ues a�d the ��de�’s f��w fie�d str��g�y depe�ds �� the accurate predicti�� �f the f��w separati�� a�d the three di�e�si��a� i�teracti�� betwee� wa!e a�d �uter f��w the SPIV data c��tai�s va�uab�e data t� va�idate the si�u�ati��. The� agai� the �u�erica� data �f the three di�e�si��a� si�u�ati�� ca� be used t� ide�tify a�d trace bac! f��w structures that appear withi� the tw� di�e�si��a� PIV p�a�es.

1 I�tr ducti �

Due t� i�creasi�g i�p�rta�ce �f fue� efficie�cy a�d further reducti�� �f C62 e�issi��s the aer�dy�a�ic perf�r�a�ce �f pr�ducti�� cars c��ti�ues t� be a !ey aspect i� research a�d deve��p�e�t. After pr�gress has a�ready bee� �ade especia��y i� the fie�d �f �ightweight structures� the aer�dy�a�ic p�te�tia� f�r decreasi�g fue� c��su�pti�� re�ai�s. F�r i�pr�vi�g the aer�dy�a�ic perf�r�a�ce� a better u�dersta�di�g �f a rea� f��w fie�d is �a�dat�ry. This fie�d ar�u�d a car bei�g a b�uff b�dy i� gr�u�d effect ca� be characteri)ed by three+di�e�si��a� f��w separati�� �r reattach�e�t a�d v�rtex f��w. T� a high degree the drag �f a b�uff b�dy depe�ds �� the wa!e structure 7Huch�]. Depe�di�g �� the rear ge��etry the wa!e itse�f is a superp�siti�� �f �ift i�duced ���gitudi�a� v�rtices �f variab�e i�te�sity a�d a �arge separated recircu�ati�� fie�d� the s� ca��ed dead water. It is a high�y turbu�e�t dissipative fie�d i�c�udi�g u�steady v�rtex f��w a�d brea!d�w� 7Eber)]. The u�derb�dy� whee�s a�d whee�h�uses ca� c��tribute as �uch as 50% t� the t�ta� drag� si�ce they have a str��g i�pact �� the �ai� f��w features as we��. Thus they ca���t easi�y be �eg�ected. Ge�eric c��figurati��s ca� �ead t� a very differe�t f��w fie�d which �a!es it difficu�t t� tra�sfer resu�ts �i!e depe�de�cies a�d para�eters t� ��re c��p�ex c��figurati��s such as pr�ducti�� cars.

The appeari�g structures withi� this c��p�ex f��w ar�u�d the detai�ed ��de�� which is prese�ted� ca� ���y be res��ved with a high+qua�ity ve��city fie�d �easure�e�t tech�ique such as Stere�sc�pic Partic�e I�age Ve��ci�etry (SPIV). This study re�ates the res��ved wa!e structures t� their ge��etric �rigi� a�d t� their c��tributi�� t� �vera�� drag. The data is used t� va�idate the

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�u�erica� s��uti�� d��e usi�g the i�c��pressib�e u�steady 6pe�F6A�@ s��ver pis�F�a�. Si�ce the f��w a�d the i�tegra� c�efficie�ts are se�sitive t� the c�rrect predicti�� �f the wa!e features� the Stere�sc�pic PIV data c��prehe�ds crucia� i�f�r�ati�� f�r a precise f��w fie�d va�idati��.

2 Experi�e�ta� Setup

Experi�e�ts have bee� c��ducted i� the i�+h�use c��sed retur� at��spheric ��w speed tu��e� A�UBB with a 1.3 x 1.3 �C c��sed test secti��. With a ��))�e c��tracti�� rate �f 4.9 the turbu�e�ce �eve� i� the test secti�� was �easured t� be TuE0.2% at UE53 �.s. F�r the experi�e�t the free strea� ve��city was set t� UE50�.s with a c�rresp��di�g Rey���ds Fu�ber �f Re E 3.0 x 106 based �� the ��de�’s ch�rd+�e�gth �f 0.885� 7Je�gthE0.885�� WidthE0.482� (�irr�r t� �irr�r)� HeightE0.362�].

The 1�4 sca�ed tu��e� ��de� �f the curre�t ge�erati�� �f the V��!swage� up! was �a�ufactured �f FECUR6F a�d a�u�i�u� by CFC �i��i�g a�d �f p�astic ��du�es usi�g rapid pr�t�typi�g. The ��de� is sh�w� i� figure 1. The detai�ed u�derb�dy i�c�udes the fr��t a�d rear ax�e� exhaust pipi�g� ta�!� rear suspe�si�� a�d spare whee� recess. The ��de� is a�s� equipped with side �irr�rs. Via f�ur struts the ��de� was c���ected t� the 6+c��p��e�t ba�a�ce L�E4 sitti�g be��w the b�tt�� wa�� �f the tu��e�. Each �easure�e�t cyc�e was ru� at a sa�p�i�g rate �f 50H) rec�rdi�g 1000 sa�p�es. U�der�eath the sp�itter p�ate the struts c���ecti�g the ��de� with the ba�a�ce are passed thr�ugh FACA0012 pr�fi�es which h��d the sp�itter p�ate a�d c�ver the f�rce struts. The ba�a�ce is p�aced i� a pressure cha�ber c���ected t� the u�der pressure at the tires. The b��c!age ca�cu�ated with the ��de�’s fr��ta� area is ab�ut 9%.

a) b)

c) d)

Fig 1 Wi�d tu��e� setup� a) ��de� side viewM b) ��de� fr��t viewM c) Detai�ed U�derb�dyM d) Wi�d tu��e�

setup i�c�udi�g ��de�� sp�itter p�ate� wi�d tu��e� b�tt�� wa�� a�d 6+c��p��e�t ba�a�ce

A���g the surface� especia��y i� the ce�ter p�a�e� 50 static pressure taps are i�tegrated. Severa�

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pressure taps were a�s� p�aced a���g the rear sp�i�er. A PSI 8400 DTC pressure �easuri�g syste� was used that was ru��i�g at a sa�p�i�g rate �f 3H) averagi�g 200 sa�p�es. The pressure tubes c���ecti�g the pressure taps a�d the �easuri�g syste� were ru� thr�ugh the tread shuff�es �f the fr��t tires. The stag�ati�� pressure was �easured with a Pra�dt� pr�be p�siti��ed upstrea� �f the ��de�. 6i� visua�i)ati�� was d��e usi�g f�u�resce�t pai�t �� the ��de�’s surface a�d the trac!. Theref�re the ��de� had a b�ac! pai�t c�ati�g t� ge�erate a g��d c��trast t� the red c���r pai�t. After a steady f��w resu�t has bee� �btai�ed the tu��e� was tur�ed �ff. T� i�pr�ve visibi�ity u�travi��et �ight was used.

Stere�sc�pic PIV �easure�e�ts were c��ducted �easuri�g three differe�t YO+i�age p�a�es ��r�a� t� the free strea�. The i�age p�a�es were ��cated at X+p�siti��s 1.25x.c� 1.35x.c a�d 2.05x.c behi�d the ��de�. The i�age di�e�si�� was set t� 460x410�� a�d was p�siti��ed cr�ssi�g the sy��etry p�a�e by 80�� i� �rder t� res��ve p�ssib�e asy��etry �f the f��w d�w�strea� �f the ��de�. A JaVisi�� Stere� PIV syste� was used i�c�udi�g tw� I�agerPr�X11� ca�eras featuri�g a CCD i�age se�s�r with a res��uti�� �f 4032x2688 pixe�s a�d a Jitr�� d�ub�e pu�sed Fd�YAG Jaser (Fa��T) with a ���i�a� �utput e�ergy �f 180�Q at 532�� wave�e�gth. Due t� adva�tages app�yi�g �ie’s �ight scatteri�g the�ry a f�rward+scatteri�g setup was ch�se� 7Raffe� et a�]. Refer t� Figure 2 f�r a� �verview �f the PIV i�age p�a�es. Upstrea� the wi�d tu��e� ��))�e DEHS Tracer partic�es were added t� the sett�i�g cha�ber. Their si)e was ab�ut 1µ� ge�erated with pressuri)ed air thr�ugh �as!i� ��))�es. F�r pr�cessi�g JaVisi��’s Davis7.2 s�ftware was used. F�r ca�ibrati�� a p�a�ar target was shifted app�yi�g the pi�h��e ca�era ��de�. Ti�e betwee� pu�ses was �i�ited by the �ut+�f+p�a�e c��p��e�ts �f the ve��city �f the �uter f��w as we�� as the bac!f��w withi� the wa!e. Thus ∆t was set equa� t� 13µs at a speed �f 50�.s. Each �easure�e�t cyc�e i�c�uded 2000 d�ub�e fra�e i�ages f�r each ca�era. Due t� ��w i�te�se ref�ecti��s �� the trac! a �as! was app�ied. F�r the vect�r fie�d c��putati�� stere� cr�ss+c�rre�ati�� was app�ied usi�g �u�ti+pass ru��i�g tw� passes at 128x128 pixe� wi�d�w si)e a�d 3 passes at 64x64 pixe� si)e. The �ver�ap was a�ways set t� 50%.

Fig 2 Stere�sc�pic PIV setup sh�wi�g the ��cati�� �f the 3 i�age p�a�es

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Due t� the rea�istic fr��t �f the ��de� the f��w is turbu�e�t� thus �a�i�ar+turbu�e�t tra�siti�� d�es ��t have a� i�pact �� the f��w fie�d. I� �rder t� �i�i�i)e the err�r because �f the ��de� �therwise sitti�g withi� the tu��e� b�u�dary �ayer a sp�itter p�ate was used as a trac!. Refer bac! t� figure 1 d) f�r a� i��ustrati�� �f this setup.

3 �u�erica� Setup

Fu�erica� si�u�ati��s have bee� d��e usi�g the �pe� s�urce s�ftware pac!age 6pe�F6A�@ Versi�� 2.1.1. The u�structured �esh with 15.1 �i�. p�i�ts was ge�erated usi�g the i�tegrated �esher b��c��esh a�d sappyHex�esh. Especia��y the v��u�e d�w�strea� �f the ��de� has a high res��uti�� i� �rder t� res��ve the wa!e structures a�d dy�a�ics i� ��re detai�. A �esh c��verge�ce study has bee� carried �ut. F�r si�u�ati��s the i�c��pressib�e u�steady RAFS f��w s��ver pis�F�a� was ch�se� usi�g a )er� pressure �ut�et as we�� as a ve��city i��et set t� 50�.s. A�s� the !+ω SST turbulence model was used. Visc�us wa��s were ��de��ed usi�g a wa�� fu�cti�� appr�ach app�yi�g the c��ti�u�us Spa�di�g �aw �f the wa��. The u�steady si�u�ati��s with ti�e steppi�g �f 0.00001s were ru� f�r 1s rea� ti�e. The �u�erica� d��ai� is represe�ti�g the wi�d tu��e� a�d is sh�w� i� figure 3�

Fig 3 Fu�erica� wi�d tu��e� d��ai�

F�te that i� the �u�erica� setup ���y the upper ha�f �f the sp�itter p�ate (c���red i� b�ue) is represe�ted. The ��wer wa�� upstrea� as we�� as d�w�strea� �f the sp�itter p�ate is treated as a� Eu�er wa��.

4 Resu�ts

A Stere sc pic PIV !easure�e�ts

The f����wi�g figures 3 t� 5 sh�w the resu�ti�g Stere�sc�pic PIV vect�r fie�ds f�r the three i�age p�a�es at x.c E1.25� x.cE1.35 a�d x.cE2.05. Figure 4 a�d 5 a) each sh�w the characteristic wa!e that f�r�s behi�d a b�uff b�dy. The �arge reverse f��w area is c�ear�y visib�e i� b�th figures but starti�g t� get �ess i�te�se d�w�strea� i� figure 5 a�d has va�ished i� figure 6 a). Due t� the b��c!age the f��w fie�d ar�u�d the ��de� is first acce�erated. The dece�erati�� �f the �uter f��w d�w�strea� �f the ��de� ca� a�s� be see� by c��pari�g the ve��city p��ts. This ca� a�s� be see� i� the pressure distributi�� sh�w� be��w. A�� p��ts i�dicate that the s�a�� asy��etry (i.e. exhaust pipe a�d gas��i�e ta�!) �f the u�derb�dy d�es ��t have a sig�ifica�t i�pact �� the wa!e si�ce the wa!e see�s t� be sy��etric. 6� the right ha�d side �f figures 4 t� 6 the x+V�rticity is p��tted.

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Withi� the first i�age p�a�e at x.cE1.25 there is a� �ver�ay �f differe�t f��w features �i!e the A+pi��ar v�rtex �r the separati�� at the whee�h�uses. The residua� �f the �eft A+pi��ar v�rtex ca� be ��cated at appr�xi�ate�y (Y +100� O 320) a�d is c�ear�y visib�e because �f the disti�ctive x+V�rticity sp�t with c�u�terc��c!wise r�tati�� a�d a�s� because �f the ���e�tu� ��ss at this p�siti�� i� the �eft picture. The v�rtex is sti�� prese�t ��e p�a�e d�w�strea� i� figure 6 but has diss��ved at the �ast p�a�e. This v�rtex ca� be traced bac! t� the vehic�e’s A+pi��ar usi�g the �u�erica� data� which is sh�w� �ater.

Fig 4 Stere�sc�pic PIV P�a�e 1 at x.cE1.25� a) x+Ve��cityM b) x+V�rticity

Fig 5 Stere�sc�pic PIV P�a�e 2 at x.cE1.35� a) x+Ve��cityM b) x+V�rticity

Fig 6 Stere�sc�pic PIV P�a�e 3 at x.cE2.05� a) x+Ve��cityM b) x+V�rticity

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J�cated at (Y +200� O 100) i� figure 4 there is a��ther sp�t �f c�u�terc��c!wise r�tati��� that ca� be traced bac! t� the separati�� ab�ve the whee�h�use. It has a���st c��p�ete�y diss��ved i� the �ext p�a�e si�ce it bec��es part �f the recircu�ati�g f��w. The A+pi��ar v�rtex i� c��paris�� is ��t rea��y affected because it is ��re �r �ess sitti�g ab�ve the wa!e withi� the �uter f��w regi��. The x+V�rticity p��t �f figure 5 sh�ws that a�� the structures that �ccurred i� the vici�ity �f the base ��w have diss��ved. 6��y tw� g��ba� ��w i�te�se ���gitudi�a� v�rtices �f the �ai� f��w re�ai� at this p�siti��.

B Pressure !easure�e�ts

Figure 7 sh�ws the pressure distributi�� at the sy��etry p�a�e �f the ��de�. The differe�t �i�es represe�t the upper a�d ��wer part �f the ��de� as we�� as the trac! a�d the wi�d tu��e� cei�i�g.

Fig 7 Static pressure distributi�� i� the sy��etry p�a�e (YE0��) �f the ��de� f�r upper side a�d

u�derb�dy� trac! a�d wi�d tu��e� cei�i�g

First at the fr��t bu�per there is the stag�ati�� p�i�t (Pressure tap �u�ber 4). 6� the upper side the f��w the� acce�erates �ver the h��d sh�wi�g a sucti�� pea! (F�. 7) bef�re there is a��ther area �f stag�ati�� at the ��wer part �f the wi�dshie�d (F�. 10). Subseque�t�y the f��w agai� acce�erates �ver the upper part �f the wi�dshie�d a�d r��f with the highest sucti�� pea! (F�. 13) t� fi�a��y sh�w a ��derate dece�erati�� t�wards the rear e�d �f the sp�i�er. At the u�derb�dy the f��w sh�ws a high acce�erati�� bef�re passi�g the ��wer fr��t sp�i�er ge�erati�g a high d�w�f�rce at the fr��t ax�e (F�. 1). After passi�g the fr��t ax�e the f��w is detached a�d a steady dece�erati�� �ccurs up t� the rear bu�per (F�. 43).

Because �f the ��de� there is a si�i�ar behavi�r �f the pressure distributi�� at the trac! as we have f�r the u�derb�dy �f the ��de�. The stag�ati�� area i� fr��t �f the ��de� is captured as we�� as the acce�erati�� �f the f��w be��w the fr��t sp�i�er. Far d�w�strea� the pressure g�es bac! up c��se t� the freestrea� va�ue. A�s� the static pressure �easured at the cei�i�g rec�vers fr�� the

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decrease a�d reaches the )er� �eve� �f the freestrea� at x.c E 2.0 appr�xi�ate�y ��e ch�rd �e�gth behi�d the ��de�.

C 'i� visua�i)ati �

Figure 8 prese�ts the surface f��w visua�i)ed with �i�. The �eft picture sh�ws the wa!e a�d the bac!f��w c��se t� the base. Behi�d the ��de� tw� v�rtex c�res appear �� the trac! sh�wi�g that the wa!e is g�ver�ed by a� �pe� h�rse sh�e v�rtex rather tha� by a c��sed ri�g v�rtex typica� f�r axis sy��etric b�uff b�dies. A�s� �� the �eft a�d right side �f the trac! the h�rse sh�e v�rtex starti�g at the fr��t �f the ��de� is visib�e. The �i� visua�i)ati�� a�s� gives very i�p�rta�t i�f�r�ati�� ab�ut the exact p�siti�� �f the separati�� �i�es at the rear part. F�r the si�u�ati�� it is esse�tia� that these �i�es �f separati�� are predicted c�rrect�y.

Fig 8 6i� visua�i)ati��� a) Wa!e recircu�ati�� area sh�wi�g the �ai� h�rse sh�e v�rtexM b) Disp�ace�e�t effect �f side �irr�r a�d A+pi��ar v�rtex

I� the right picture �f figure 8 the f�r�ati�� �f the A+pi��ar v�rtex is sh�w�. A�s� the separati�� behi�d the side �irr�r has a str��g effect �� the surface strea��i�es at the sides.

D Drag c �p �e�ts i�tegrati �

A���g with the ba�a�ce+�easured t�ta� f�rces the i�dividua� drag c��p��e�ts withi� the f��w a�d at a specific x+p�siti�� such as v�rtex drag ca� be ca�cu�ated by i�tegrati�g the acquired SPIV fie�d data usi�g the c��tr��+v��u�e appr�ach �eth�d 7A�ders��]. The t�ta� drag caused by ���e�tu� ��ss is ca�cu�ated with�

∫∫

−⋅=

∞outletS

ssMomentumLoD dSU

u

Ac

2

2

, 11

(1)

With this f�r�u�ati�� the drag c�efficie�t is a �easure f�r the ���e�tu� that is ��st withi� the c��tr�� v��u�e� where a PIV �easure�e�t p�a�e is said t� be the �ut�et face. We assu�e t� have idea� c��diti��s at the i��et face �f the c��tr�� v��u�e with a freestrea� ve��city �f 50�.s a�d a freestrea� pressure at )er�. Pressure �easure�e�ts a���g the trac! a�d cei�i�g i� figure 7 i�dicate that the static pressure at PIV �easure�e�t p�a�e 3 (x.c E 2.05) has c��e bac! up c��se t� this freestrea� va�ue (figure 7). At this x+p�siti�� i�terp��ati�� gives a cp E 0.025 at the trac! a�d a cp E

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17th International Symposium on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 07-10 July, 2014

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+0.026 at the cei�i�g. Averagi�g �f these tw� va�ues w�u�d �ead t� a���st )er�. Thus the pressure ca� be �eg�ected f�r the drag differe�ce i�tegrati�� at p�a�e 3. At p�a�e 1 a�d 2 �� the �ther ha�d� a c�rrecti�� f�r the pressure w�u�d be �ecessary.

The v�rtex drag is direct�y re�ated t� the i�+p�a�e f��w c��p��e�ts withi� the �easure�e�t p�a�e. Agai� there is the assu�pti�� �f idea� c��diti��s with �� i�+p�a�e f��w at the i��et face �f the c��tr�� v��u�e. The t�ta� v�rtex c�efficie�t �f the �easure�e�t p�a�e agai� is ca�cu�ated with a� i�tegra� �ver the e�tire �ut�et face �f the c��tr��+v��u�e.

∫∫

+⋅=

∞∞outletS

VortexD dSU

w

U

v

Ac

2

2

2

2

,

1 (2)

I�tegrati�� is d��e usi�g Tecp��t 360 Versi�� 2013. The resu�t �f the i�tegrati�� is sh�w� i� figure 9. The figure sh�ws a c��paris�� �f the differe�t drag c�efficie�ts. The c�efficie�t deter�i�ed with the ba�a�ce �atches the i�tegrated surface pressure �f the �u�erica� s��uti�� quite we��. The PIV i�tegrati�� �eth�d u�der predicts the drag c�efficie�t. The i�tegra� is se�sib�e t� the c�rrect freestrea� ve��city.

0-389

0-017

0-410

0-196

0-378

0-018

0-325

0-006

0-415

0-0

0-1

0-2

0-3

0-4

0-5 Exp 6/0ba�a�ce f rce

Exp Base Drag

Exp SPIV ! �e�tu�Drag P�a�e 1

Exp SPIV V rtex DragP�a�e 1

Exp SPIV ! �e�tu�Drag P�a�e 2

Exp SPIV V rtex DragP�a�e 2

Exp SPIV ! �e�tu�Drag P�a�e 3

Exp SPIV V rtex DragP�a�e 3

�u�erica� Si�u�ati �Pressure I�tegrati �

Fig 9 C��paris�� �f drag c�efficie�ts deter�i�ed with differe�t pr�cedures

A�s� the v�rtex drag due t� i�+p�a�e f��w is rather s�a�� c��pared t� the ���e�tu� ��ss. This is i� agree�e�t with previ�us studies sh�wi�g that i�duced drag te�ds t� be s�a�� f�r ��st c��ve�ti��a� r�ad vehic�es 7Va� Da�].

The base drag ca� be ca�cu�ated with the surface pressure �easure�e�ts at the base a�d the base area. It appears t� be appr�xi�ate�y ha�f �f the t�ta� drag.

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E C �paris � with �u�erica� data

I� this part the experi�e�ta� data is c��pared with the �u�erica� si�u�ati��. 6� the �eft side �f figure 10 the PIV data �f p�a�e 1 is sh�w�. The right side sh�ws a s�ice �f the averaged u�steady RAFS ca�cu�ati�� at the sa�e x+p�siti��. The d�w�wash �f the upper f��w see�s t� be s�ight�y �ver predicted �eadi�g t� a higher �egative )+ve��city i� the �idd�e p�a�e. 6vera�� there is a g��d agree�e�t betwee� the experi�e�ta� a�d �u�erica� data. Figure 11 agai� sh�ws the x+v�rticity at PIV p�a�e 1.

Fig 10 x+Ve��city� C��paris�� �f stere�sc�pic PIV P�a�e 1 at x.cE1.25 a�d the averaged f��w fie�d s�ice �f

the u�steady RAFS si�u�ati��

6� the right side a ra�d�� ti�e step �f the u�steady �u�erica� si�u�ati�� is sh�w�. I� this picture it is p�ssib�e t� trace bac! s��e �f the f��w features that appear i� the 2D PIV p�a�e. Especia��y the A+pi��ar v�rtices sitti�g �� t�p �f the wa!e are prese�t. A�s� the ��w i�te�se ���gitudi�a� structures ev��vi�g fr�� the base are visib�e.

Fig 11 x+V�rticity� C��paris�� �f stere�sc�pic PIV P�a�e 1 at x.cE1.25 a�d a ra�d�� ti�e step �f the

u�steady RAFS si�u�ati�� (view shifted t� the �eft)

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4 C �c�usi �

I� this paper a� experi�e�ta� study �f a detai�ed r�ad vehic�e ��de� was prese�ted. Due t� the c��p�ex f��w stere�sc�pic PIV �ffers the p�ssibi�ity t� get a �uch better i�sight a�d t� c��ser i�vestigate the f��w structures a�d wa!e dy�a�ics. F�r further research based �� detai�ed ��de�s it is esse�tia� t� exa�i�e the f��w fie�d i� ��re detai� f�r better c��prehe�si�� rather tha� ���y a�a�y)i�g si�g�e i�tegra� va�ues. It was sh�w� h�w differe�t �easuri�g tech�iques i� c��bi�ati�� with �u�erica� data ca� describe a three+di�e�si��a� f��w fie�d a�d that si�g�e structures ca� be extracted. Si�g�e drag c�efficie�ts were c��pared sh�wi�g a g��d �atch �f experi�e�ta� f�rce �easure�e�ts a�d �u�erica� surface pressure i�tegrati��. A�s� the ca�cu�ated base drag c�efficie�t is withi� the expected ra�ge. The PIV i�tegrati�� sh�ws better agree�e�t with the f�rce �easure�e�t c��se t� the base. The c�efficie�t decreases further d�w�strea�. 6vera�� the v�rtex drag is s�a�� c��pared t� the t�ta� drag a�d the v�rtices start t� dissipate d�w�strea� �f p�a�e 2. The �u�erica� si�u�ati�� c��fir�s the structures f�u�d with the SPIV �easure�e�ts.

Refere�ces

•A�ders�� QD (2001) Fu�da�e�ta�s �f Aer�dy�a�ics. �cGraw+Hi��� B�st��.

•Eberz TQ (2001) BeitrTge )ur 3D+LFO+Aer�dy�a�i!. Sha!er+Ver�ag� Aache�.

• Huch� WH (2005) Aer�dy�a�i! des Aut���bi�s. ViewegUTeub�er+Ver�ag� Wiesbade�.

•Raffe� �� Wi��ert C� Were�ey S� L��pe�ha�s Q (2007) Partic�e I�age Ve��ci�etry+ a Practica� Guide. Spri�ger+Ver�ag� Ber�i�� Heide�berg� Few Y�r!.

•Va� Da� CP (1999) Rece�t experie�ce with differe�t �eth�ds �f drag predicti��. Pr�gress i� Aer�space Scie�ces 35� 751+798.