Initial Turbulence Effect on Jet Evolution With and Without Tonal Excitation

The e f f e c t o f i n i t i a l turbulence l e v e l on the development o f a j e t and on t h e s u s c e p t i b i l i t y o f t h e j e t t o d i s c r e t e tone e x c i t a t i o n was exper imen ta l l y i nves t i ga ted . Turbulence i n t e n s i t y was var ied, over t h e range 0.15 t o 5 percent, by us ing screens and g r i d s placed upstream o f an 8.8 cm didi i ie ie i nozzie . Top-hat mean v e l o c i t y F7 boundary l a y e r s ta tes were ensured i n a l l cases; A t h e tu rbu lence spect ra were broadband. It was found, c o n t r a r y t o e a r l i e r repo r t s , t h a t t h e n a t u r a l j e t decay remained e s s e n t i a l l y unchanged f o r v a r y i n g i n i t i a l turbulence. For a f i x e d ampl i tude o f t h e t o n a l e x c i t a t i o n , i nc reas ing t h e i n i t i a l turbulence damped o u t the growth o f t h e i n s t a b i l i t y wave; as a r e s u l t , the e x c i t a b i l i t y , assessed f rom t h e mean v e l o c i t y decay on t h e a x i s , was found t o d imin ish. However, the degree o f damping i n t h e a m p l i f i c a t i o n o f t he i n s t a b i l i t y wave was on ly s l i g h t compared t o t h e l a r g e increase i n t h e i n i t i a l turbulence. The j e t w i t h 5 pe rcen t turbulence cou ld be measurably a l t e r e d by e x c i t a t i o n w i t h a v e l o c i t y pe r tu rba t i on ampl i tude as l i t t l e as 0.25 percent o f t h e j e t v e l o c i t y . The ampl i tude e f f e c t data i n d i c a t e an upper bound o f t h e e x t e n t t o which a j e t could be exc i ted , and thus i t s plume shortened, by t h e plane wave, s i n g l e frequency e x c i t a t i o n . y i e l d i n g a nominal ly laminar boundary layer , re-emphasizes the profound e f f e c t of i n i t i a l boundary l a y e r s t a t e on j e t e v o l u t i o n as w e l l as on i t s e x c i t a b i l i t y . Th is j e t decayed t h e f a s t e s t n a t u r a l l y , and consequently, i t was t h e l e a s t e x c i t a b l e i n s p i t e o f i t s turbulence being the l e a s t . N 0 p r o f i l e s w i t h approximately i d e n t i c a l i n i t i a l Ari ddditionai d d t d b e t w i t h IIW y r l d or t r ip , I n t r o d u c t i o n The i n f l u e n c e o f an a r t i f i c i a l l y imposed e x c i t a t i o n on j e t c h a r a c t e r i s t i c s i s f a i r l y well documented.1-6 I n these s tud ies, i t was found t h a t e x c i t a t i o n cou ld e i t h e r enhance o r suppress tu rbu lence depending on t h e Strouhal number, ampl i tude and mode of e x c i t a t i o n , as w e l l as the Reynolds number and t h e i n i t i a l boundary l a y e r s ta te . What has remained p r a c t i c a l l y unknown i s t h e e f f e c t of va ry ing i n i t i a l turbulence on the j e t development, and consequently, on t h e s u s c e p t i b i l i t y of t h e j e t t o d i s c r e t e tone e x c i t a t i o n . One o f t h e main d i f f e r e n c e s between a j e t engine exhaust and a l abo ra to ry model j e t i s i n t h e i n i t i a l core turbulence. Turbulence i n t e n s i t y i n f u l l s c a l e Tu rbo je t (Olympus 593) and Turbofan ( R B 211) en ines have been repor ted t o be 3 t o 1 5 percent,$ w h i l e t h a t i n a l a b o r a t o r y j e t i s t y p i c a l l y 0.5 pe rcen t o r l ess . Thus, w h i l e cons ide r ing the p o s s i b i l i t y o f a r t i f i c i a l c o n t r o l of Ilreal l $ f * I l jets, a quej:ion natura::y can such a j e t be exc i ted , and i f so, t o what ex ten t? J e t e x c i t a t i o n experiments have been c a r r i e d ou t t a k i n g i n t o account t h e e f f e c t o f var ious o t h e r f a c t o r s l i k e of f round nozz le sha ese8 ef fects11 and c o m p r e s s i b i l i t y effect^.^ On t h e e f f e c t o f i n i t i a l turbulence, a s e t o f experiments have been c a r r i e d ou t by Sov ie t researchers, e.g. References 12 and 13. Reference 13 repor ted a s i g n i f i c a n t l y f a s t e r j e t decay w i t h i nc reas ing turbulence. However, t h e r e a r e c o n f l i c t i n g t rends i n the repo r ted data. Although no t discussed, a g r e a t e r j e t decay i s i n d i c a t e d a t 5 percent i n i t i a l t u rbu lence than a t 10 percent i n i t i a l turbulence. Thus. a c a r e f u l recheck seemed t o be i n order . Also, t h e paper does n o t document o r d iscuss t h e nature ( spec t ra ) o f t h e turbulence, t h e e x i t v e l o c i t y p r o f i l e and t h e s t a t e o f t h e boundary l aye r . Oh and Bushnell14 emphasized t h e necess i t y o f such data w h i l e comparing t h e i r numerical r e s u l t s on a m ix ing w i t h another experiment by Vinogradov. e t a l . (see Ref. 14). The o b j e c t i v e o f t h e present experiment was t o gather a systemat ic s e t o f data on t h e e f f e c t o f i n i t i a l core tu rbu lence on t h e e v o l u t i o n o f an ax isymnetr ic j e t and i t s response t o acous t i c e x c i t a t i o n . I n t h e p r e l i m i n a r y stage, cons iderable e f f o r t was spent i n f a b r i c a t i n g and t e s t i n g t h e g r i d s t h a t would produce var ious turbulence i n t e n s i i i e s b u t leave t h e top-hat e x i t p r o f i l e unchanged. I n t h e end, f o u r cases were d i s t o r t e d v e l o c i t y p r o f i l e s . 9 heat f l u x . 1 8 ground 1 I A ..-..I-.^ .....;...*L....... r*..r-..,. r r n A i + i r n r layer >PI cau UIIU~I v a t i u u a I I cc-a 1.1 caw GVIIU I c I V I I ~ * For reasons remaining unknown, a un i fo rm square mesh g r i d w i t h l a r g e blockage produced an ax isymnetr ic b u t nonuniform mean v e l o c i t y p r o f i l e a t t h e e x i t . Fo r example, such a g r i d w i t h 80 percent blockage y i e l d e d 15 percent h ighe r mean v e l o c i t y on t h e pe r iphe ry than t h a t on t h e a x i s .