Role of strut shape on the generation of internal side force and moments in A supersonic nozzle with strut insertion through diverging wall

Abstract

Cold flow experiments were conducted on a convergent-divergent (C-D) nozzle with a strut inserted through the nozzle diverging wall to study the internal wall pressure distribution. The objective of the work was to calculate the internal side force and hence the pitching moment generated as a consequence of the side insertion of the strut through the nozzle wall. The generation of side force/pitching moment would be useful for the development of a potential alternative thrust vector control system of flight vehicles employing a supersonic nozzle. The strut was inserted at a distance of 2/3rd of the diverging section length ( L d) of the nozzle from the throat. Two cross sectional shapes of the strut i.e. square with V-notch and semi-ellipse were employed in the experiments. Eight wall pressure ports each on the strut side and on counter strut side of the nozzle axis were used to acquire the pressure data. The design exit Mach number ( M d) corresponding to isentropic flow conditions of the nozzle was 1.84 with an area ratio of 1.48. The cold flow experiments were conducted at three nozzle pressure ratios (NPRs) which were 3.4,5 (both corresponded to over-expansion) and 6.9 (under-expansion). For each strut shape and at a given NPR, strut height ( h s) was varied to study the internal wall pressure distribution. The maximum height of the strut was restricted to the radius of the local cross section of the nozzle (r) where the strut was inserted. From the wall pressure ( p w) distribution, two-dimensional side force ( C s) and moment coefficients ( C m) were calculated. The variation of these coefficients with h s was plotted and the effect of the strut shape for each operational NPR was studied. These variations with h s in respect of square-notch shape and semi elliptical shape were also compared with variations corresponding to square shape available in the literature. The variations of C s and C m were highly nonlinear and trends of both the variations were similar. It was observed from these variations that the semi-elliptical strut shape exhibited a distinct behavior which was in contrast to two other shapes. Variation in h s resulted in both positive and negative values of coefficients in respect of square with V-notch and square shaped struts, whereas in case of semi-elliptical shape mostly positive values of the coefficients were observed. The maximum positive magnitude of C s was observed at strut heights which were 27% and 57% of the local radius of the nozzle cross section (where the strut was inserted) for square with V-notch and semi-elliptical struts respectively.