Effect of contact geometry on the friction of acrylamide hydrogels with different surface structures

Polyacrylamide (PAAm) hydrogels with brush-covered or crosslinked surfaces were produced and their tribological behavior was studied over a wide range of sliding speeds for two different contact geometries: sphere-on-flat and flat-pin-on-flat. Irrespective of the contact geometry, the brushy hydrogel surfaces displayed up to an order of magnitude lower coefficients of friction μ (COF) compared to the crosslinked surfaces, even achieving superlubricity (μ < 0.01). In general, a hydrogel sphere showed a lower coefficient of friction than a flat hydrogel pin at a similar contact pressure over the entire range of sliding speeds. However, after normalizing the friction force by the contact area, the shear stress of hydrogels with either crosslinked or brushy surfaces was found to be similar for both contact geometries at low speeds, indicating that hydrogel friction is unaffected by the contact geometry at these speeds. At high sliding speeds, the shear stress was found to be lower for a sphere-on-flat configuration compared to a flat-pin-on-flat configuration. This can be attributed to the larger equivalent hydrodynamic thickness due to the convergent inlet zone ahead of the sphere-on-flat contact, which presumably enhances the water supply in the contact, promotes rehydration, and thus reduces the friction at high sliding speeds compared to that measured for the flat-pin-on-flat contact.

Three-Dimensional Simulation of Wind-Driven Ventilation Through a Windcatcher With Different Inlet Designs

Windcatcher is an effective natural ventilation system, and its performance depends on several factors including wind speed and wind direction. It provides a comfortable and healthy indoor environment since the introduced fresh air decreases the moisture content and reduces the pollutant concentration. Since the wind speed and its direction are generally unpredictable, it is important to use special inlet forms and exits to increase the efficiency of a windcatcher. In this study, computational fluid dynamics (CFD) modeling is implemented using ansys fluent to investigate the airflow entering a three-dimensional room through a windcatcher with different inlet designs. Three designs are studied which are a uniform inlet, a divergent inlet, and a bulging-convergent inlet. The airflow pattern with all inlets provided adequate ventilation through the room. With all the applied wind velocities (1, 2, 3, and 6 m/s) at the domain's inlet, the divergent inlet shape has captured the highest airflow through the room and provided higher average velocity at 1.2 m high enhancing the thermal comfort where most of the human occupancy occurs. With 6 m/s wind velocity, the divergent inlet has captured 2.55% more flow rate compared to the uniform inlet and 4.70% compared to the bulging-convergent inlet, and it has also provided an average velocity at 1.2 m high in the room of 7.16% higher than the uniform inlet and 8.44% higher than the bulging-convergent inlet.

Three Dimensional Simulation of the Effect of Windcatcher’s Inlet Shape

Windcatcher has been used over centuries for providing natural ventilation using wind power, it is an effective passive method to provide healthy and comfortable indoor environment. The windcatcher’s function is based on the wind and on the stack effect resulting from temperature differences. Generally, it is difficult for wind to change its direction, and enter a room through usual openings, the windcatcher is designed to overcome such problems since they have vertical columns to help channel wind down to the inside of a building. The efficiency of a windcatcher is maximized by applying special forms of opening and exit. The openings depend on the windcatcher’s location and on its cross sectional area and shape such as square, rectangular, hexagonal or circular. In this study the effect of the inlet design is investigated to achieve better air flow and increase the efficiency of windcatchers. To achieve this, CFD (computational fluid dynamics) tool is used to simulate the air flow in a three dimensional room fitted with a windcatcher based on the different inlet designs. The divergent inlet has captured the highest air flow with a difference of approximately 3% compared to the uniform inlet and 5% difference compared to the bulging-convergent inlet.

Effect of Wind Speed on Ventilation Flow Through a Two Dimensional Room Fitted With a Windcatcher

Natural ventilation is the process of supplying and removing air through an indoor space by natural means. Windcatcher has been used over centuries for providing natural ventilation using wind power, it is an effective passive method to provide healthy and comfortable indoor environment by decreasing moisture content in the air and reducing pollutants concentration. The windcatcher’s function is based on the wind and on the stack effect resulting from temperature differences. Generally, it is difficult for wind to change its direction, and enter a room through usual openings, the windcatcher is designed to overcome such problems since they have vertical columns aimed at helping wind to channel down to the inside of a building. The efficiency of a windcatcher is maximized by applying special forms of opening and exit. The openings depend on the windcatcher’s location and on its cross sectional area and shape such as square, rectangular, hexagonal or circular. In this study the effect of different wind speeds on the total air flow captured by different inlet designs is investigated. To achieve this, CFD (computational fluid dynamics) tool is used to simulate the air flow in a two dimensional room fitted with a windcatcher applying wind speeds from 1 m/s up to 14 m/s and based on different inlet designs such as a uniform inlet, a divergent inlet and a bulging-convergent inlet.

Effect of Windcatcher’s Inlet Shape on Ventilation Flow Through a Two Dimensional Room

Natural ventilation is the process of supplying and removing air through an indoor space by natural means. Windcatcher has been used over centuries for providing natural ventilation using wind power, it is an effective passive method to provide healthy and comfortable indoor environment by decreasing moisture content in the air and reducing pollutants concentration. The windcatcher’s function is based on the wind and on the stack effect resulting from temperature differences. Generally, it is difficult for wind to change its direction, and enter a room through usual openings, the windcatcher is designed to overcome such problems since they have vertical columns aimed at helping wind to channel down to the inside of a building. The efficiency of a windcatcher is maximized by applying special forms of opening and exit. The openings depend on the windcatcher’s location and on its cross sectional area and shape such as square, rectangular, hexagonal or circular. In this study the effect of the inlet design is investigated to achieve better air flow and increase the efficiency of windcatchers. To achieve this, CFD (computational fluid dynamics) tool is used to simulate the air flow in a two dimensional room fitted with a windcatcher based on different inlet designs such as a uniform inlet, a divergent inlet and a bulging-convergent inlet.