1、TechnicalPaper3032November 1990=B-=Experimental Investigationof Porous-FI00r Effectson Cavity Flow Fieldsat Supersonic Speeds_oyd J. Wilcox, Jr .T T-:=il2:i=i:_=:2 :T: : : : :Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Provided by IHSNot for Resa
2、leNo reproduction or networking permitted without license from IHS-,-,-NASATechnicalPaper30321990Experimental Investigationof Porous-Floor Effectson Cavity Flow Fieldsat Supersonic SpeedsFloyd J. Wilcox, Jr.Langley Research CenterHampton, VirginiaNationalAeronautics andSpace AdministrationOffice of
3、ManagementScientific and TechnicalInformation DivisionProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-SummaryAn experimental investigation was conducted
4、 todetermine the effectiveness of a passive-venting sys-tem to modify the flow field over a rectangular-boxcavity at supersonic speeds. The passive-venting sys-tem consisted of a cavity that had a porous floor witha vent chamber beneath the floor. The vent chamberallowed high pressure at the rear of
5、 the cavity to ventto the low-pressure region at the forward section ofthe cavity, thus modifying the cavity flow field. Twowind-tunnel tests (one drag and one pressure test)were conducted to determine the effectiveness of thispassive-venting system.The wind-tunnel model consisted of a rectangular-b
6、ox cavity mounted in a flat plate. For the drag test,the cavity was mounted on a one-component balancesuch that only the drag of the cavity was measured.The cavity height remained constant throughout theentire test, and the cavity length was varied withblock inserts. Solid-, porous-, and a combinati
7、on ofsolid- and porous-floor configurations were tested forcomparison. The tests were conducted at Mach num-bers of 1.60, 1.90, 2.16, and 2.86 and at a constantReynolds number of 2 x 106 per foot. The resultsshowed that the porous floor was very effective inmodifying the cavity flow field as evidenc
8、ed by alarge reduction in the cavity drag. The data alsoshowed that the porosity near the cavity midlengthdid not significantly affect the venting process; thisresult suggested that other methods (e.g., an array oftubes) could be used to modify the cavity flow field.In order to define completely the
9、 cavity flow field, asecond test was conducted to measure pressures inthe cavity. The same flat-plate model (except witha new cavity that had pressure orifices located alongthe cavity floor, on the forward- and rear-cavity faces,and on the vent chamber floor) was used. The resultsshowed that the por
10、ous floor modified the cavity flowfield to an intermediate type flow field. The resultsalso showed that stores mounted in the cavity did notdiminish significantly the effectiveness of the porous-floor venting system.IntroductionOne of the most important mission goals for mil-itary fighter aircraft i
11、s to carry and launch weaponssuccessfully. For supersonic cruise fighter aircraft, in-ternal store carriage has received considerable inter-est because of the reduced aircraft radar cross sectionand reduced store carriage drag compared to externalstore carriage arrangements. The successful launchof
12、weapons from internal weapons bays (cavities) re-quires a knowledge of the cavity flow field to preventstore separation problems. This paper examines amethod for modifying the flow field of certain cavi-ties which typically causes adverse store separationcharacteristics and thus possibly improves th
13、e sepa-ration characteristics of stores from these cavities.Although this paper focuses primarily on cavitiesused for weapons bays, other uses for cavities includeobservation ports on aircraft and recessed areas forfins before deployment on wraparound fin missiles.Existing data available in the lite
14、rature (rcfs. 1to 4) show that three basic types of cavity flow fieldsexist at supersonic speeds. These flow fields are com-monly referred to as closed-, transitional-, and open-cavity flows. The type flow field which exists for agiven cavity depends primarily on the cavity length-to-height L/h rati
15、o. Cavity flow fields with L/h _13are generally referred to as closed-cavity flows and arecharacterized by a flow that separates at the cavityleading edge, expands into the cavity, attaches to thecavity floor, and then separates and exits ahead of thecavity rear face (fig. 1). The corresponding pres
16、suredistribution shows a low-pressure region at the for-ward section of the cavity as the flow separates andexpands into the cavity, an increase in pressure as theflow impinges on the cavity floor, a pressure plateauas the flow passes along the cavity floor, and an in-crease in pressure as the flow
17、compresses as it turnsto exit the cavity ahead of the rear face. Keepingthe cavity height constant and decreasing the cav-ity length will shorten the pressure plateau regionon the cavity floor. When the pressure plateau re-gion is eliminated and the pressure increases steadilyfrom the forward sectio
18、n of the cavity to the rear ofthe cavity, the flow field generally is referred to as atransitional-cavity flow, and the cavity L/h is gen-erally between 10 and 13. If the cavity length is de-creased more so that L/h _ 12, although the effect becomesgreater as L/h increases. This result suggests that
19、as the amount of venting increases, the height of thevent chamber restricts the vent chamber flow.A comparison of schlieren photographs for boththe solid- and porous-floor cavities (L/h = 17.500) isshown in figure 22. This comparison illustrates theeffect of the porous floor on the cavity flow field
20、. Inthe solid-floor photographs, the impingement shockthat forms as the flow expands into the cavity andattaches to the cavity floor and the shock that isformed as the flow exits the cavity are clearly visible.The porous-floor photographs show a complete elim-ination of this entire shock-wave system
21、; this elimi-nation again suggests that the flow field is probablytypical of open-cavity flow (fig. 1).Data also were obtained with adhesive tape par-tially covering the porous floor. The tape was ar-ranged symmetrically about the cavity midlength(fig. 23) to determine if the porosity near thecavity
22、 midlength had a significant effect on thecavity flow field. Figure 24 shows data for acavity with L/h = 17.500 and a vent chamberheight of 0.30 in. The results show a steady de-crease in the cavity drag as the percentage of floorarea with porosity increases. The solid-floor cav-ity drag was reduced
23、 by one-half when approxi-mately 35 percent of the floor area was porous.When more than 50 percent of the floor area wasporous, the additional drag reduction obtained wassmall. Therefore, the porosity near the cavity mid-length does not significantly affect the cavity flowfield (i.e., the porosity o
24、n the forward and rear sec-tions of the cavity floor has the largest effect). Thisresult suggests the possibility that other methods(e.g., an array of tubes) could be used to directlytransport the high-pressure air at the rear of the cav-ity to the low-pressure region at the forward part ofthe cavit
25、y and still obtain the same results as for theporous floor.The results of the drag test showed that thepassive-venting system was effective in modifying thecavity flow field and thus reduced the cavity drag.Because the resUlts ofthedrag study could notdefine completely the porous-floor cavity flow f
26、ield,a second wind-tunnel test was conducted to measurethe pressures inside the cavity and thus define theflow field of the porous-floor Cavity.Pressure TestsBecause the drag results of the porous-floor cavityshowed very little effect of vent chamber height, allthe pressure data were obtained with a
27、 vent chamberheight of 0.15 in. (i.e., the smallest vent chamberheight studied during the drag tests). As discussedin the “Model Description“ section of this report, thefloor porosity was 11.2 percent during the pressuretests instead of 7.9 percent used during the dragtests. Both floor porosities mo
28、dified the cavity flowfield, although no direct comparison of the effect ofporosity was conducted during this investigation.Solid-floor results. Figure 25 shows the cen-terline pressure distributions on the cavity forwardface, floor, and rear face along with the forward- andrear-face lateral distrib
29、utions for the solid-floor cav-ity. Data are presented for selected cavity L/h ra-tios to illustrate closed flow, transitional flow just be-fore switching (transitional-closed flow), transitionalflow just after switching (transitional-open flow), andopen flow. Note that transitional-cavity flow hasb
30、een divided into two types of flow: transitional-closed and transitional-open flow. Transitional-closed flow is the same flow field that is referredto as transitional flow in the “Introduction“ sectionof this paper. Transitional-open flow occurs as thecavity L/h is decreased slightly from that requi
31、redfor transitional-closed cavity flow. For this type flowfield, the flow separates and expands into the cavity,is turned through a series of compression waves (butdoes not attach to the cavity floor), and then exitsat the rear of the cavity. Sketches of the flow fieldsand the terms used to describe
32、 the various types offlow fields discussed in the remainder of this paperare shown in figure 26. For Mach numbers of 1.60,1.90, and 2.16, each of the previously discussed flowtypes occurs at the same L/h at each Mach number.At a Mach number of 2.86, transitional-closed andtransitional-open flows occ
33、ur at L/h ratios slightlysmaller than those for the other Mach numbers. Thecenterline pressure distributions agree with the re-sults discussed in the “Introduction“ and thereforegive credence to this experimental setup.All the lateral pressure distributions on the for-ward and rear faces of the cavi
34、ty are symmetricalabout the cavity centerline. The rear-face data showthat a large increase in pressure occurs at the outsideedges of the cavity for the closed- and transitional-closed-flow cases. This pressure increase is caused bythe impingement of vortices, which are formed alongthe cavity side e
35、dges as the flow expands into thecavity. This flow phenomenon has been documentedin references 1 and 5. The pressure distributions forProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-transitional-openflow(exceptat Moo = 2.86) showa slight pressure inc
36、rease at the cavity edges; thispressure increase is probably the result of the im-pingement of a weak vortex that formed from therelatively smaller expansion (as compared to the ex-pansion for the closed- and transitional-closed-flowcases) at the cavity leading edge. In contrast, thepressure distrib
37、utions for the open-flow case showa slight pressure decrease at the cavity edges. Thepressure magnitudes on the rear face decrease withdecreasing L/h, and the largest decrease occurswhen the flow switches from transitional-closed totransitional-open flow. The forward-face distribu-tions for closed a
38、nd transitional-closed flows showa slight increase in pressure at the cavity centerlinefor Mo_ = 1.60, 1.90, and 2.16, although the mag-nitude of the pressure increase is reduced as Moo isincreased. The pressure distributions for open andtransitional-open flows are nearly constant across thecavity w
39、idth. The magnitude of the pressures in-creases as L/h decreases; the largest increase occurswhen the flow switches from transitional-closed totransitional-open flow.The pressure difference on the forward and rearfaces gives an approximation of the cavity drag for agiven L/h. The closed-flow and tra
40、nsitional-closed-flow cases have the largest pressure differences andconsequently the largest drag. A large decrease in thepressure difference, which is due to the decrease inpressure on the rear face and an increase in pressureon the forward face, occurs as the flow switches fromtransitional-closed
41、 to transitional-open flow. Theseeffects also were seen in the drag data discussedpreviously.Solid- and porous-floor comparisons. Com-parisons of the solid- (closed-) and porous-(transitional-open) floor cavity pressure distributionsfor L/h = 17.500 and the pressure distributions fora solid-floor ca
42、vity with transitional-open flow areshown in figure 27. The centerline pressure distribu-tions show the typical closed-cavity flow field for thesolid-floor cavity with L/h = 17.500. The porous-floor data show that the flow field has switched fromclosed flow to a flow field that is similar to the one
43、found for a solid floor with transitional-open flow;however, the magnitudes of the pressures on the floorare slightly different. On the forward and rear facesof the cavity, the magnitudes and trends of the cen-terline and lateral pressure distributions are nearlythe same for the porous floor and the
44、 solid floor withtransitional-open flow. Hence, the porous-floor cav-ity with L/h = 17.500 has a flow field that is similarto transitional-open flow.Figure 28 shows a comparison of the cavityfloor and vent chamber floor (figs. 5 and 13) pres-sure distributions for a porous-floor cavity withL/h = 17.
45、500. The pressure difference between thecavity floor and the vent chamber floor indicates thatat the rear of the cavity, air passes from the cav-ity to the vent chamber; near the cavity midlength,little or no air passes between the cavity and thevent chamber; and at the forward section of the cav-it
46、y, air passes from the vent chamber to the cavity.The pressure distribution on the vent chamber floorshows a decrease in pressure from the rear of the cav-ity toward the cavity midlength where the pressurereaches a minimum before increasing at the forwardsection of the cavity. This distribution sugg
47、ests thatthe flow velocity at the rear and forward sectionsof the vent chamber (where the air enters and ex-its the vent chamber) is slower than that near thecavity midlength (where the flow velocity reaches amaximum).These results (figs. 27 and 28) indicate that theporous-floor flow field is simila
48、r to the hypotheticaldescription discussed in the Introduction except thatthe flow changes to transitional-open flow rather thanopen flow. This change suggests that the porous-floorflow field has reached an equilibrium state such that alarge enough pressure differential exists between theforward and
49、 rear sections of the cavity to allow theventing process to continue. If the porous-floor flowfield switched to completely open flow, essentiallyno pressure differential would exist to maintain theventing process. This change to open flow wouldresult in the flow field switching back to closed flowand thus starting an oscillating behavior. If thisoscillating behavior were present, data points wouldnot repeat readily, and schlieren photographs of t
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