محمد حججی

Among the different methods of water supply, extracting water from atmospheric fog has recently received more attention from scientific communities due to its advantages and unique features. In this method, water droplets are trapped and separated from the air using metal meshes. The main challenge facing this technology is the low efficiency of water harvesting from humid air. To overcome this issue, the idea of using a metal porous collector has been tested in the presence of an electrostatic field. Parameters such as the porosity percentage of the collector, airflow speed, electric field intensity, and the distance between the emitter and the collector (field distance) on the water harvesting efficiency were studied experimentally. In the range of investigated porosities, the efficiency of the fog collector has its maximum value with a porosity of 95.6%. Also, reducing the field distance increases the efficiency. Investigating the effect of electric field voltage on the efficiency of the device showed that with the increase in voltage from 15 to 24 kV, the efficiency increases from 21% to more than 42%. At this voltage, the phenomenon of voltage saturation has occurred, so that a further increase in voltage does not have much effect on the efficiency. Investigating the effect of flow speed showed that the maximum efficiency of the device was achieved at a speed of 1.1 m/s.

Downburst storms cause severe destruction by creating intense and unstable downdrafts. On the other hand, due to the difference in their structure from atmospheric boundary layer storms, it is essential to study and understand these flows under different conditions. Therefore, this study the effects of the impact angle of the downburst and the structure installation angle relative to the surface flow on a cube-shaped model investigates dynamically. The model is placed in front of the downburst in four angles of the storm colliding with the surface(θ), in two directions of the surface flow relative to the structure(α), and in the radial range of X/D=±1.5. Also, the ratio of horizontal displacement speed of this storm(VR) is considered to be 0.06 and 0.12. The results show that the increase of θ and VR caused the location of the maximum pressure coefficient to shift from the central point of the flow meeting the surface to the downstream. Also, increasing α has reduced the range of pressure and force changes by about 25% on the model. In addition, it was found that dynamic downburst caused stronger impacts on the structure and generally, these strong impacts occurred immediately after the downburst passed over the structure.

A Downburst can produce divergent outflow wind on the ground surface, which is different from the behavior of atmospheric boundary layer flows. In this research, the effects of downburst on a cube-shaped structure in two different directions of flow (α), four different ground surface angels relative to the downburst direction (θ), and different radial distances (X/D) relative to the downdraft center were investigated by a simulator that was made for this thunderstorm. Simulation of this flow is created by a blower whose task is to uniformize the flow created by the fan embedded behind it. The velocity and turbulence intensity of flow was measured at different X/Ds. also, the distribution of pressure coefficient on the sides of the model was measured at the X/D locations. In addition, a good agreement has been observed between the data comparison of this study and previous studies. It was observed that at the center of the downburst for all θs, the structure has a positive pressure coefficient along its sides. By moving away from the center of the storm, the flow behavior is similar to the boundary layer flows. By increasing θ, it was found that the difference of pressure coefficient between the windward side relative to the roof and the backward sides, increased, which in the worst case has changed by about 80%. By examining the direction of flow to the model, it was found that the force coefficients when the model is at α=45°, are about 35% less than when the model is at α=0°. Finally, it was found that at X/D=1, the maximum force coefficient is applied to the structure.

In this study, due to the importance of controlling the thrust vector of beveled nozzles, the effects of secondary injection on the output flow deviation from the convergent-divergent beveled nozzle have been investigated experimentally. For this purpose, three beveled nozzles with beveled angles of 25, 45, and 65 degrees have been used. Also, to control the direction of the output flow from the nozzle, a convergent injector has been installed to discharge the secondary flow perpendicular to the center direction of the nozzle at 0.8 divergent nozzle length. The main parameters studied in this research are the effects of changing the stagnation pressure of the main nozzle and the secondary flow and changing the beveled nozzle angle. The output flow deviation from the nozzle in different conditions is extracted from the shadowgraph images, and the results are presented. The results show that the performance of beveled nozzles with a low beveled angle with secondary injection is very good and the flow control in beveled nozzles up to 45 angle is much more favorable than high beveled angle nozzles. As the beveled angle increases, the nozzle performance decreases Strongly, So that the rate of reduction of flow deflection in 45 beveled nozzle compared to the 25, was close to 3%. While this deflection in the 65 beveled nozzle is reduced to 83%.

The purpose of this study is to experimentally evaluate the performance of two models of new type of contact pneumatic grippers. One of them is two-dimensional and the other one is axisymmetric, which uses a dual throat ejector to handle limp and porous materials. In two-dimensional device, slotted plates with different slot width was used to investigate the effect of slot width and total pressure of inlet flow on the performance of the gripper. In the axisymmetric gripper, perforated rings with different hole diameters was used to evaluate their effect and the effect of high pressure inlet flow on the performance of the device. Results show that in both grippers, by increasing inlet pressure, the induced pressure and suction force to hold limp materials increases. Also, regardless of the gripper mass flow rate consumption, the axisymmetric gripper with higher mass flow consumption creates more force and suction pressure, but the performance of the two-dimensional gripper in producing suction force is much better by considering the mass flow rate consumption.

In this study, the effect of protuberance on the thrust vector of a supersonic jet was investigated as a new method in thrust vector control. For this purpose, a convergent-divergent nozzle was designed and fabricated. This nozzle is such that the nominal Mach number in the nozzle exit in full expansion condition is 2. The wall of the nozzle is equipped with pressure holes to measure pressure variations. Also, there is a hole on the nozzle wall to apply a protuberance inside the nozzle. Pressure sensors for pressure measurement and also the Schlieren system are used to check the outlet flow field. The total pressure in all experiments is constant and equal to NPR=6.6. Three-dimensional and multi-block numerical code is used for flow modeling. Also, the turbulence model k-ε, RNG is used to model the nozzle flow. An unstructured mesh has been used for modeling the flow field within the nozzle and the outside domain. The results of this study show that the depth of penetration of the protuberance in the flow field has a significant effect on the amount of deviation and even the direction of the jet stream exited from the convergent-divergent nozzle. The maximum deviation of the jet outlet from the nozzle is 9.8 degrees, which is equal to a penetration ratio of 0.4. In addition, these results indicate that with the increase in protuberance penetration within the nozzle, the nozzle axial thrust has slightly decreased.

In this study, the effect of a cylindrical protuberance on the thrust vector of a supersonic jet was investigated as a new method in thrust vector control. For this purpose, a convergent-divergent (C-D) nozzle was designed and constructed. This nozzle is such that the Mach number is its nominal output in full expansion conditions 2. The wall of the nozzle is equipped with pressurized holes to measure pressure variations. Also, there is a duct wall in the nozzle wall to apply a protuberance inside the nozzle. Pressure sensors are used for pressure measurement and also the schlieren system IS used to check the outlet flow field. The total pressure of the compartment is constant at all tests and is 5.7 bar. The results of this study show that the depth of penetration of the protuberance in the flow field has a significant effect on the amount of deviation and even the direction of the deviation of the jet stream exited from the C-D nozzle. The maximum jet outlet flow from the nozzle is 5.7 degrees, which occurred at a rate of H/D*=0.4. In addition, these results indicate that with the increase in bulge penetration within the nozzle, the nozzle axial thrust has slightly decreased.

In this study, the effect of a cylindrical protuberance on the thrust vector of a supersonic jet was investigated as a new method in thrust vector control. For this purpose, a C-D nozzle was designed and constructed. The nozzle exit Mach number is 2. The wall of the nozzle is equipped with pressure holes to measure pressure variations. Also, there are several holes in the divergence portion of nozzle wall to apply a protuberance inside the nozzle. Pressure sensors for pressure measurement and also the Schliern system are used to check the outlet flow field. The nozzle pressure ratio in all experiments is constant and in two cases is equal to NPR=6.6 and NPR=9. The protuberance is installed in the nozzle divergence section, at position X⁄L=0.6,0.7,0.8,0.9 and with a constant penetration ratio of H⁄D^* =0.2. The results of this study show that using the protuberance can control the angle of the thrust vector. Also, installing location X⁄L=0.9 is the best position which, in this case the angle of the trusted vector reaches 3.1 degrees. Also, the results reveals that the change in the nozzle pressure ratio in different installing positions has different effects on the thrust vector angle and axial thrust losses.

In this study, the effect of a cylindrical protuberance on the thrust vector of a supersonic jet was investigated as a new method in thrust vector control. For this purpose, a convergent-divergent nozzle was designed and constructed. This nozzle is such that the Mach number is its nominal output in full expansion conditions 2. The wall of the nozzle is equipped with pressurized holes to measure pressure variations. Also, there is a duct wall in the nozzle wall to apply a protuberance inside the nozzle. Pressure sensors for pressure measurement and also the schlieren system are used to check the outlet flow field. The total pressure of the compartment is constant at all tests and is 5.7bar. The results of this study show that the depth of penetration of the protuberance in the flow field has a significant effect on the amount of deviation and even the direction of the deviation of the jet stream exited from the convergent-divergent nozzle. The maximum jet outlet flow from the nozzle is 5.7degrees, which occurred at a rate of H/D*=0/4. In addition, these results indicate that with the increase in bulge penetration within the nozzle, the nozzle axial thrust has slightly decreased.

The effect of counterflow jet through an extended nozzle on reducing aerodynamic drag is analyzed by using a combined method. Flow field is simulated around a hemispherical body in a free stream with Mach 4. The results are reached by providing a 3D solver and applying the complete form of Navier-Stoke and energy equations along with modified shear stress transport model. Appropriate numerical validation has been made by comparing the surface pressure distribution in the zero pressure ratio of jet to free-stream and drag on the nose at a pressure ratio of 0 to 3. Four nozzles were used to analyze the effect of extending. The results show that the nozzle extensions have a significant effect on the wave drag after changing the shape of the bow shock. In a given pressure ratio, the effect of injected jet from the extended nozzle over the reduction of the nose is higher than that of direct jet injection from the nose. The effect is visible in all pressure ratios. Furthermore, a limited increase in the pressure ratio over a fixed length of the extended nozzle has led to a further reduction of total drag. However, in the higher pressure ratios, the linear increase of the retro jet has led to an increase in the total drag on the nose. The results also show that increasing the nozzle length in a constant pressure ratio leads to an increase in the depth of jet penetration and a larger reduction of total drag.

Effects of secondary sonic jet injection in divergent part of supersonic nozzle on flow field structure and thrust vector control performance has been numerically analyzed. Three dimensional multi-blocks extended numerical code has been used to model the complexity of turbulence flow by k-ω SST model. Structured computational domain has been applied and initial results of simulation validated with previous experimental results. The obtained numerical results are compared with the experimental ones, and the outcome shows acceptable agreement between the two. Different injection power generates by varying the injection surface and pressure ratio with respect to throat pressure. Injection power increment make changes in performance and also sometimes it lowers the performance. In the current research aside from complete complex flow features description, allowable power range to increase system performance has been presented. In this range, increasing the injection mass flow rate, decreases the amplification factor, but increases the deflection angle and axial thrust augmentation as most important performance parameters. Out of estimated range for allowable mass power injection, performance parameters different behavior differently that shows a drastic drop in performance.

An experimental study has been performed to investigate the flow physics of an under-expanded axisymmetric supersonic jet exhausting transversely from a flat plate into a subsonic compressible cross flow. The goal was to study the effects of boundary layer thickness ratio (δ/D) and the jet to free stream dynamic pressure ratio (J) on surface pressure. The resulting measurements distinctly show that both the magnitude and the width of the positive and the negative pressure coefficient regions in front and behind the jet increase with J. In addition, the side region of the jet affects the jet plume and as J increases, the pressure coefficient decreases at this region. The data acquired upstream of the jet reveal that, with increasing δ/D, the magnitude of the positive pressure coefficient region decreases, While the recovery of the back pressure downstream of the jet moves in the vicinity of the nozzle exit.

T h e p r o b l e m o f j e t i n t o a c r o s s f l o w i s a c l a s s i c p r o b l e m i n f l u i d m e c h a n i c s a n d h a s m a n y e n g i n e e r i n g a p p l i c a t i o n s. M u c h w o r k h a s b e e n d o n e i n t h e f i e l d o f j e t i n t o a c r o s s f l o w, h o w e v e r, n o t m u c h o f i t h a s b e e n i n v o l v e d w i t h t h e i n t e r a c t i o n s o f s u p e r s o n i c j e t s i n c o m p r e s s i b l e s u b s o n i c c r o s s f l o w, e s p e c i a l l y t h e i r i n t e r a c t i o n w i t h t h e c o n t r o l s u r f a c e s l o c a t e d d o w n s t r e a m. I n t h i s s t u d y, t h e e f f e c t s o f a f i n l o c a t e d i n t h e v o r t i c a l f l o w f i e l d g e n e r a t e d b y t h e p e r p e n d i c u l a r e x h a u s t i n g o f a s u p e r s o n i c j e t i n t o a h i g h s u b s o n i c c r o s s f l o w, h a v e b e e n i n v e s t i g a t e d. S o m e o f t h e i n d i v i d u a l a c h i e v e m e n t s o f t h i s r e s e a r c h a r e p a r a m e t r i c s t u d i e s o f j e t i n t o f r e e s t r e a m d y n a m i c p r e s s u r e r a t i o (J) a n d t h e l o c a t i o n o f f i n s. F o r t h i s p u r p o s e, a f l a t p l a t e e q u i p p e d w i t h p r e s s u r e h o l e s, a f i n w i t h a N A C A 0012 a i r f o i l, a s p e c i a l r a k e t o d e t e r m i n e t o t a l p r e s s u r e a n d a s p e c i a l h a r d w a r e t o p r o v i d e s u p e r s o n i c j e t w e r e d e s i g n e d, m a n u f a c t u r e d a n d u s e d. T h e f r e e s t r e a m a n d t h e e x h a u s t e d j e t M a c h n u m b e r s w e r e h e l d c o n s t a n t (0.7 a n d 2.5, r e s p e c t i v e l y). T h e r e s u l t s r e l a t e d t o t h e i n t e r a c t i o n s b e t w e e n t h e j e t, t h e f r e e s t r e a m, a n d t h e f i n d o w n s t r e a m (t h e c a s e w h e r e t h e f i n i s a t t h e p l a n e o f s y m m e t r y) s h o w t h a t, w h e n t h e f i n i s l o c a t e d i n t h e n e a r f i e l d o f t h e j e t, t h e s t r e n g t h o f t h e c o u n t e r r o t a t i n g v o r t e x p a i r (C R V P), d u e t o i n t e r a c t i o n w i t h t h e f i n, i n c r e a s e s. H o w e v e r, w h e n t h e f i n i s l o c a t e d i n t h e f a r f i e l d, f e w c h a n g e s a r e o b s e r v e d. F o r t h e c a s e w h e r e t h e f i n i s i n t h e n e a r f i e l d w i t h a l a t e r a l d i s t a n c e f r o m t h e p l a n e o f s y m m e t r y, s t r o n g i n t e r a c t i o n b e t w e e n t h e f i n a n d o n e o f t h e v o r t i c e s o f C R V P i s o b s e r v e d. I n s o m e c a s e s, t h i s v o r t e x a n d t h e r e l a t e d j e t p l u m e w e r e a l s o b u r s t. F o r c a s e s w h e r e t h e f i n i s i n t h e f a r f i e l d, w i t h a l a t e r a l d i s t a n c e f r o m t h e p l a n e o f s y m m e t r y, t h e f l o w f i e l d i s n o t a f f e c t e d b y t h e p r e s e n c e o f t h e f i n a n d t h e v o r t i c e s p a s s n e a r t h e f i n w i t h o u t m u c h c h a n g e.