An Analysis of Drag Force Components and Structures in the...
Transcript of An Analysis of Drag Force Components and Structures in the...
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
17th International Symposium on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 07-10 July, 2014
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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�
17th International Symposium on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 07-10 July, 2014
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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
17th International Symposium on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 07-10 July, 2014
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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.
17th International Symposium on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 07-10 July, 2014
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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
17th International Symposium on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 07-10 July, 2014
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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
17th International Symposium on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 07-10 July, 2014
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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
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.
17th International Symposium on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 07-10 July, 2014
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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)
17th International Symposium on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 07-10 July, 2014
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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.