\ci^rfi\-6('i LtMHm V < NATIONAL ADVISORY COMMITTEE ^ FOR AERONAUTICS TECHNICAL MEMORANDUM 1361 CONCERNING THE FLOW ON RING-SHAPED COWLINGS Part XIII THE INFLUENCE OF A PROJECTING HUB By D. Kuchemaim Translation of "ZWB Untersuchungen und Mitteilungen Nr. 3144.' Washington UNIVERSITY OF FLORIDA October 1953 DOCUMENTS DER^RTMENT " STON SCIENCE LIBRARY 117011 r ..iLLE.FL 32611-7011 USA . 1^5 6 (oil 1^ NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS TECHNICAL MEMORANDUM I36I CONCERNING THE FLOW ON RING-SHAPED COWLINGS Part XIII THE INFLUENCE OF A PROJECTING HUB* By D. Kuchemann Abstract: The influence of thickness and length of a hub projecting from an inlet opening was investigated on one of the two new classes of circulELT cowls reported in NACA TM I36O. Outline: I. STATEMENT OF THE PROBLEM II. RESULTS III. SUMMARY IV. REFERENCES I. STATEMENT OF THE PROBLEM In some applications, there arises the problem of scooping a given air quantity out of the free stream, as, for example, through a circular opening in the case of an annular radiator. It could also be visualized, however, that such an encircling inlet opening might be provided in the installation of special propulsion units, for instance in the fuselage. One may regard such an annular inlet as an inlet with projecting hub for which the flow phenomena to be expected axe already known to a great extent. One knows, moreover, from various model tests that, with such a projecting hub, one can reduce the excess velocities on the outside of the circular cowl but has to accept on the other hand a decreased total pressure in the entrance opening. (See for instance ref. 1.) More accu- rate data for evaluation of the usability of this inlet arrangement (from which the actual numerical amount of the separate phenomena in various arrangements could be estimated) are still lacking, however. The properties of two new classes of circular cowls have been investi- gated in reference 2. We shall in the present report observe more closely the influence of the hub on one of these circular cowls. We select for this purpose the least contracted cowl of class IV with Fj./F =0.6 and "Ueber die Stromung an ringformigen Verkleidungen. XIII. Mitteilung; Der Einfluss einer vorgezogenen Nabe." Untersuchungen und Mitteilungen Nr. llkk (ZWB). NACA TM 1361 provide it with six different hub bodies which differ in thickness and length. 1 (Compare figs. 1 and 2.) The hubs obstructed 50, 65, and 80 per- cent of the entrance cross section F„' of the circular cowl and were, beginning from the foremost point of the cowl, circular -cylindrical in its interior. For the short hubs, a nose in the form of a semiellipsoid of an axis ratio 1:2 was affixed so that the length Z^ of the projecting part of the first hub was exactly equal to the diameter 2 Rj^. In a sec- ond series, the hub was lengthened frontward by a cylindrical piece of the length of one diameter so that ^^/^ % became 2. The investigations are limited to pressure distribution and mass-flow measurements. II. RESULTS In the following figures, we indicated only the most significant results which show the important phenomena; the detailed measuring results may be had from the AVA. From figures 3 ^^^ k, one can see how the suction peaks at the out- side of the cowl are lowered by the guiding action of the hub which shifts the stagnation point farther outward and reduces or eliminates separation (or welling-over) of the cowl boundary layer. For the case without hub the pressure distributions show the unfavorable properties of cowlings with slight rounding of the lip; the cowl lip appears, from the pressiore distributions, to become increasingly rounded out with growing hub size. Accordingly, the excess velocities decrease shar-ply (figs. 5 and 6). The magnitude of this reduction depends on the inlet velocity ratio; it is highest when the entering flow is completely throttled and can, natiirally, hardly be ascertained at all when free-stream velocity prevails in the entrance cross section. How far this drop in excess velocity increases the critical drag-break Mach niraiber cannot be determined, since the character of the pressure distribution also is changed entirely by the hub. Furthemiore, the reduction of the excess velocities is particularly noticeable in case of oblique approach flow due to the hub as can be seen from figures 7 and 8. The decrease of the excess velocities may be explained partly on the basis of potential theory; we recall that thrust forces must act on the outside of the cowl (compare refs. 1 and 3) and that a certain area must be put at their disposal if the negative pressures are not to drop below certain values. If the hub projects very far from the fairing, it cannot be put to use for the application of the thrust forces so that only the frontal area of the cowl Fg^ - Fg' could be considered as thrust area. As is discussed in detail in reference 1, a decisive factor for the As no symbol list was included in the German text, a list has been compiled by the NACA reviewer and is given in an appendix. NACA TM 1361 3 magnitude of the excess velocity is in addition to the inlet velocity- ratio, ■v-cj/Vq, the ratio between the entrance area and the thrust area, that is, the quantity (P-g' - Fj^jAFg^ - Fjjj, which replaces the con- traction, Fg/Fg^, in the hubless inlet. This quantity decreases more and more with increasing thickness of the hub, that is, the contraction of the cowl increases and the cowl becomes more favorable. The maximum excess velocity '^joax/'^o ^^i^^ would be obtained for vg = if the cowl had a constant pressure distribution (Ruden's entrance cone of minimum contraction) is calculated from For the arrangements investigated we obtain the values; Hub ratio Contraction Excess velocity %/Fe' 1 - Fjj/Fe' ^max/^o ^°^ ^E = ^ V^e' - %/Fe' 0.60 1.58 .50 .1^3 1.33 .65 .3U I.2U .80 .23 l.lU These values of excess velocity are not attained with the cowl used since it is not a Ruden minimum form; for the long hub with a larger radius of curvature at the lip where the presuppositions probably come true, the minimum values are exceeded by approximately 10 percent. There is a second reason, however, which is responsible for the reduction of the excess velocities: (that is) the flow separation at the hub. This phenomenon is, moreover, expressed in the fact that at the entrance the full total pressure is not attained. For two different flow quantities, the total pressure distributions at the entrance (begin- ning of the cylindrical part) for the various arrangements in rectilinear flow are shown in figure 9} figures 10 and 11 show the values averaged over the entrance cross section referred to the stagnation pressure of the approach flow which is designated as inlet efficiency T]-g. One can see that the losses rise with increasing hub thickness and length and reach noticeable amounts in case of the thickest hub. The numerical NACA TM 1361 values themselves, however, can give only a clue, since they are cer- tainly caused, to a high extent, by the small model size (maximum outer diameter of the hub 200 mm, free-stream velocity about kO m/s). It should further be pointed out that the inlet efficiencies depend also very considerably on the shape of the hub and are, for Instance, essen- tially less favorable in forms which are thickened ahead of the inlet and are again contracted at the entrance. (Compare, for instance ref. 1.) In case of oblique approach flow, the separation phenomena become still less clear. In figure 12, one can see measurements for the most extreme arrangement in a vertical center section which shows the greatest vari- ations. The entrance losses for static condition for the hub Fn/^e' =0.5 may be taken from reference 2. For the thicker hubs, the inflow for static conditions takes place practically without losses. III. SUMMARY In a circular cowl, the influence of a hub body projecting from the inlet opening was investigated; the reduction of excess velocities on the outside of the cowl and the amount of total pressure losses in the inlet as a function of thickness and length of the hub were ascertained by measurements. The tests are visualized for the application in annular radiators and the installation of special propulsion units; in the latter case, such an arrangement would have to show many constructive and other advantages in order to prevail over the customary forms of Installation since one does not gain anything in frontal area and always has to accept noticeable entrance losses. Translated by Mary L. Mahler National Advisory Committee for Aeronautics. NACA TM 1361 APPENDIX SYMBOLS^ F-,/F contraction of cowl ^E'/Fa - %^/^a contraction of circular cowl without hub Fe' = = «V % = «r/ p ^ges Po > H <; -i-> ■ — t ^H f% X! W u d -3 U o ■f-i NACA TM 1561 (XI II ^ cr Without iub / J / (1 Figure 3.- Circular cowl IV/0.6; a = 0°; Vg = 0; with short hub ^n/2Rn = ^S pressure distribution outside. MCA TM 1361 11 Figure 4.- Circular cowl rV/0.6; a = 0°; Vg = 0; with long hub Zi>j/2Rn = 2; pressure distribution outside. 12 NACA TM 1561 2.0 1.9 1.8 V max V, '0 1.7 t 1.6 1.5 1,4 1.3 12 1.1 \ \ V \ \ \ \ \ \ \ < \ \ s. \ , V^o = x. N \ ^ ::::^4Q> s> ""^ ^ 104 — 0.6' """^ ~~~ ■^ — " — -T-" no * '^ U.o 1 0.5 06 0.7 0.8 0.9 1.0 Fn/Fc Figure 5.- Circular cowl IV/0.6; a = 0°; with short hub Jn/2Rn = 1; excess velocities outside. NACA TM 1561 13 U max t - V 1.9 I \ 1 4 8 \ \ 1.7 \ \ \ 1.6 \ \ \ A C \ \ \ 1.0 \ \ \ 7 1 /)) - 1.4 . \ x^ )- N ^ ^, V ^s *v Of. \ ^ 1.3 «• •*> r^ ^ ^N ^•J -T^— 1.2 GB 1.0 1 1 1.1 10 ■ 0.5 0.6 0.7 0.8 Q9 10 Figure 6.- Circular cowl IV/0.6; a = 0°; with long hub 2^/2% = 2; excess velocities outside. li+ NACA TM 1561 -2.2 h / -^rr - 2.1 Oi< — 1 r / V / "Without hub f / \ ^*^ S_^ J! / •^ ^ ^ ^ Z- y ^ ^ ^ * ^' " " V5 ^' p -^" / ^^ ^e/^o=0 - - \.\ l'^/l'o = 0.6 ■ 1 , ° 1 1 -5' 0' 10* a Figure 8.- Circular cowl IV/0,6; with long hub ljg/2Rj^ = 2; excess velocities outside. 16 NACA TM 1361 r-rt nv~ - — 1 ^ ^?r 8n ^v ■s ' ^ ^ "■n. ^ \ ^ ♦s. ^ ^« '■ ■^-' "^ 00 31 cr 1 f* "^-♦^ ^~c "> •-^ J-. ^ ^_ -i "d c 1 *^ ^ T -— Q. ^. 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