Ramraj Kannan

The potential applications of superhydrophobic surfaces have garnered significant interest among researchers. While numerous methods have been proposed to achieve superhydrophobicity on various materials, only a limited number are suitable in terms of process simplicity, productivity, and cost-effectiveness. In this study, a hot embossing process was employed to enable Teflon surfaces as superhydrophobic surfaces with the bio-inspired surface micro / nano features, addressing these limitations. Bioinspired dual-scale roughness patterns were created on a copper surface through sandblasting, followed by electrochemical etching. Using the patterned copper master, the reverse of dual-scale roughness patterns was subsequently transferred onto a polytetrafluoroethylene (PTFE) surface via hot embossing. By systematic variation of electrochemical etching parameters such as current density and etching time, and hot embossing parameter i.e., hot press temperature, the appropriate dual scale, i.e., micro / nano structures have been enabled on the Teflon surface to imbibe with superhydrophobic characteristics. As a result, the fabricated superhydrophobic surfaces exhibited an average static water contact angle (SWCA) of 150°. These surfaces are checked its superhydrophobic robustness by investigating the retraction dynamics of the impacting water droplet with different impact velocities. By understanding the robustness in static water droplet contact angle and retraction dynamics of the impacting drop on these surfaces, these superhydrophobic characteristics can be extended in solving ice aggregation control on airborne structures and bio-fouling control in marine structures.

This paper investigates influence of the inner wall wettability on mass flow rate and discharge coefficient (Cd) for the flow of water through a 1 mm diameter capillary tube with circular cross-section. The wettability of the inner wall of the tube was modified by dip coating of the capillary tubes in the solution made out of Trichloro(1H,1H,2H,2H-tridecafluoro-n- octyl) silane (FOTS) and n-hexane. Further, the coated capillary tube is heated to 250 ⁰C and 350 ⁰C for a time span of 2 hours to enable effectiveness of coating. Upon coating and heating to 250 ⁰C, the contact angle was incremented from 32.72° (for uncoated capillary tube), to 114.92°. At the same time, the inner surface contact angle has reduced to 67.72⁰ while heating the coated tube to 350 ⁰C. In order to analyse the effect of length to diameter ratio of the capillary tube on fluid flow behaviour, three different orifices with the length of 10mm, 15mm, and 30mm were chosen with three different wettability conditions. The increase in contact angle resulted in higher mass flow rates and Cd for all the capillary tubes considered in this study. The uncoated orifice tubes resulted in the lowest flow performance, while the orifice with a contact angle of 114.92° led to the highest efficiency, implying that reduced contact of the wall with water liquid minimizes the energy dissipation and enhances the fluid transport. The consistency in the trends which has been observed across the different Length to Diameter ratios proposes the idea that wettability modification can be used as an effective and scalable method for enhancing fluid transport systems. And by refining of this property gives precise manipulation of fluid transports in applications like microfluidics, fuel injection and bio medical devices

The investigation of Shock Wave Boundary Layer Interaction (SWBLI)-induced Separation Bubble (SB) formation and control was conducted numerically in a mixed compression intake of a scramjet engine. External compression was achieved through three successive oblique shocks formed by the forebody ramps. The intake was designed so that all three shocks converge and impinge on the leading edge of the cowl lip at an operating Mach number of 5.0. Numerical simulations were performed by solving steady, compressible 2D RANS equations using the transitional SST k–[ ] turbulence model to analyze the influence of SWBLI on supersonic intake performance. This study examined SB formation and its control by implementing a porous wall near the SB in three configurations based on the perforation relative position to the SB. The results showed that the size of the SB decreased to an acceptable level when the upstream section of the bubble was solid, and the downstream section was porous. The porous wall downstream of the separation bubble effectively reduced the bubble’s size and mitigated its negative impacts on intake performance. This study demonstrates that strategically placing porous walls can optimize the aerodynamic efficiency of scramjet engines operating at high Mach numbers. The findings provide valuable insights for designing future hypersonic vehicles, where managing SWBLI is critical for ensuring structural integrity and operational efficiency under extreme conditions.

Birds have traits that can induce better aerodynamic efficiency along with high manoeuvring capability during its flight, which could be shared with unmanned aerial vehicles for improving their aerodynamic performances. One such feature of the wing tip, i.e. the primary feathers of the birds could be an effective geometrical feature to reduce the wing tip vortices. This paper presents the bio-inspired wing tip devices, i.e. three-and four-tipped multiple winglets in reducing the strength of vortices emanating from the wing tip of the wing operating in the Reynolds number (Re) of [ ] and [ ]. Different combinations of both three- and four-tipped multiple winglets have been designed by varying the cant angle of each tip. Numerical simulations were carried out using Ansys-Fluent by solving three-dimensional Reynolds averaged Navier–Stokes formulations coupled with k-[ ] turbulence model to resolve the features of tip vortices. The simulation clearly indicates that there is a strong correlation between the size of the vortices and the aerodynamic performance parameters such as [ ], [ ] [ ], [ ]. The three- and four-tipped multiple winglets are effective in reducing vortex drag by disintegrating large strength vortex which occurs in the tip of straight wing, into few numbers of small strength vortices. When compared to straight wing, three-tipped multiple winglet with the cant angle combination of 50, 30, 10 improves the aerodynamic efficiency by 22% to 23% and the four-tipped winglet with the cant angle combination of 60, 50, 40, 30 enhances the same by 21% to 22% in the Re of [ ]. Even the longitudinal static stability has seen considerable improvement for four-tipped multiple winglets than three-tipped multiple winglets and straight wing.

This study numerically investigates the characteristics of unsteady vortex shedding phenomena behind V-gutter bluff bodies, with emphasis on their application for achieving reliable and energy-efficient combustion in air-breathing propulsion systems. The study explores the effect of the V-gutter’s semi-span angle as well as the provision of a central slit on the leading edge of the bluff body, on the modification of the wake which is critical for flame stabilization. The transient non-reacting fluid flow over the V-gutter bluff body is modelled with two discrete semi-span angles of 60° and 30° and with four central slit diameters of 1 mm, 0.75 mm, 0.5 mm, and 0.25 mm. Based on the characteristics of the wake, the study shows that the V-gutter that has a semi-span angle of 60° and a central slit of 0.25 mm or 0.5 mm would be the most effective configuration within the range of parameters considered in the study.[open-strick] [close-strick]Keywords: Wake flow, Flame stabilization, Central slit V-gutter, Bluff body, Vortex shedding.

A series of reduced-order numerical simulations on a specific bluff body type (v-gutters) in a subsonic duct flow is performed to assess the unsteady wake dynamics. Two of the v-gutter's geometrical parameters are varied: the v-gutter's base angle (θ) and the size of a slit (ξ) at the leading-edge of the v-gutter. Turbulent flow kinematics and pressure field are analyzed to evaluate the unsteadiness at a freestream Mach number of M∞=0.25 and a freestream Reynolds number based on bluff body's transverse length (L = 17.8 mm) of ReL=0.1×106. Five v-gutter angles are considered (θ, rad=π/6,π/4,π/3,5π/12,π/2) and three slit sizes (ξ, mm =0,0.25,0.5) are considered only for a particular θ=[π/6]. In general, high fluctuations in velocity and pressure are seen for the bluffest body in consideration (θ=π/2) with higher drag (cd) and total pressure loss (Δp0). However, bluffer bodies produce periodic shedding structures that promote flow mixing. On the other hand, the presence of a slit on a streamlined body (θ=π/6) tends to efficiently stabilize the wake and thus producing almost a periodic shedding structure with lower cd and Δp0. For θ=[π/6], broadened spectra in vortex shedding are seen with a peak at [fL/u∞]∼0.08. For θ≥[π/4], a dominant discrete shedding frequency is seen with a gradual spectral decay. Similarly, the effects of ξ on the θ=[π/6] case produce a discrete shedding frequency instead of a broadened one, as seen in the cases of θ≥[π/4]. The shedding frequency increases to a maximum of [fL/u∞]∼0.26 for the maximum slit size of ξ=0.5 mm. From the analysis of the x–t diagram and the modal analysis of vorticity and velocity magnitude in the wake, the peaks are indeed found to agree with the spectral analysis. More insights on the shedding vortices, momentum deficit in the wake, varying energy contents in the flow field, and the dominant spatiotemporal structures are also provided.

This study investigates the turbulent flow field downstream of V-gutters using unsteady numerical modelling. An important domain of application of the vortex shedding induced by the V-gutters is the flame stabilization in high speed combustion systems which find extensive applications in aerospace engineering. In view of this, the present study analyses the impact of the V-gutter geometry, as characterized by the included angle, on inducing vortex motion in the wake. Transient simulations are carried out for three values of the semi-span angle, α = 30°, 45° and 60°. Based on the analysis of the saddle point and the vortex shedding frequency, the study shows that an increase in span angle within this range, favours the effectiveness of the method in flame stabilization. Though the simulations are done for cold flow, the dominant mechanism of vortex shedding is adequately addressed in the analysis.