External aerodynamics CFD analysis with adequate accuracy and short turnaround time
Calculation of a probability of failure value at the beginning of its life
Design of large axial fan blade profile
Introduction :
In fluid mechanics terms, vehicles are bluff bodies travelling very close to the ground. Complex three-dimensional flow occurs around the vehicles with flow separation being very common. Such flow is typically analysed and optimized through external aerodynamics studies.External aerodynamics of sports cars, passenger cars and commercial vehicles which move at a relatively higher speeds, as such vehicles experience significant drag force which impacts performance and fuel consumption. Drag is mainly pressure drag; so, avoiding or reducing the separation is one of the main objectives of vehicle aerodynamics.
Here we are presenting a case study to depict how CFD analysis can be used to improve aerodynamics of a relatively slower speed commercial vehicle while overcoming above constraints.
Case Study:This case study is on a legacy design of auto rikshaw (3-wheeler). The body design is done in the last century and aerodynamic considerations were not the primary factors back then. If we want to convert such vehicle to electric vehicle, we need to consider aerodynamics and check how much electric power can be saved and also, if range can be increased through design improvements to the body shape.
Baseline design:
The shape of the 3-wheeler considered for this study is as shown below.
Revised design:
Based on the external aerodynamic study, the body shape is revised considering the constraints listed below.
The revised design is shown below. The features have scope for more optimization to further improve the aerodynamic performance.:
Boundary Conditions:
Both designs are analysed for aerodynamics performance at the vehicle speed of 15 m/s.
Results: Drag Coefficient (Ref. Wikipedia)
From the results, it can be observed that 17.4 % reduction in power requirement was achieved.
This will result in increase in the range of the vehicle as well.
Results:
From the above plots for baseline design, design modifications to improve the drag coefficient were carried out with following considerations:
Results: Streamlines side view:
Results: Streamlines Top view:
From these above streamlines plots, following improvements can be observed:
Conclusions :
Introduction:
If a product fails, we must repair it or replace it to continue using it and this involves costs. Gone is the era when failure costs were only considered to be the manufacturing costs. In this world, reputation costs are much higher. So, this should not mean, providing extra (costly) features with the approach “I do not want to take any chances”.
Fig-1: Ref: Brochure on rockfalls published by the Department of Mines of Western Australia.
For designers, it is pretty basic to check the strength of the final product before even manufacturing it. The strength of a product depends primarily on the two factors, i.e. specification limit and stresses generated due to normal loading (geometry).
Typical hurdles in the product design are, customers want the product to work in an environment which is beyond the specifications, on the other hand designers wants to control the manufacturing expenses to keep the product costs down. Robustness of a product can be achieved from a good design itself than a very sophisticated manufacturing process.
Traditional way to check the strength of a component is to check whether strength is more than the stress. But the new way considers the variance of specification limits as well as stress values.
Fig-2: Approaches
Case Study:
In this article, it is seen that how probabilistic calculations helped to get the real picture of a simple product. A basic probability of failure is calculated for a design which is passing in FEA i.e. in traditional approach.
Description: The geometry is a simple pipe with thick walls, having two branches as shown below. It is subjected to internal pressure loading.
Fig-3: Pipe with branches
Inputs: The detailed drawings with the manufacturing tolerances and material properties with mean yield strength value and its variance.
Approach:
Results:
Fig-4: Resultant distributions
Fig-5: FOS distribution
Conclusions:
Introduction:
In today’s world, turbomachinery is everywhere from computer cooling fan to aircraft engine. But design of one is quite a big task. They have to work at various operating points say on the planet Earth to Mars. As they are pretty fundamental in a bigger machine, in case of failure, most of the times, it is a catastrophe. Design of turbomachinery involves design of blade profile, hub, shroud etc. for aerodynamic performance as well as structural strength and manufacturability. The design features are so much interwoven that getting all of them at optimal level is a herculean task. So, by its nature it is an iterative process.
Inputs to the design are pretty basic, i.e. pressure ratio, flow requirements, prime mover speed, interface dimensions, etc.
Case Study:
It is required to design a large axial fan to lift a human being for training the parachuters. So, the size of the fan should be around 7 ft in diameter. It should create enough updraft to lift around 75 kg of human weight. The approximate air speed required to lift a human is around 300 km/hr. The air flow should not have any ‘zero/low flow air rings along the direction of flow. The flow should not rotate along with the fan. This driven by a standard motor with 1440 RPM rotational speed. The blades should be stiff enough to sustain their own weights when stationary and aerodynamic loads when operational. The fan should be placed in a vertical duct.
Description:
Fig-1: Final geometry of the fan which was meeting all the requirements
Results:
Fig-2: Pitch and blade angle variation along the radius of the blade
Fig-3 CL vs Cd graph at 3 different speeds and variation of angle of attack with fan speed and pressure ratio at a radial location
Fig-3: Baseline and optimized velocity profile on a downstream section
Fig-4: Axial Thrust, Velocity and Power consumption
